Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity - Additional Information (Detail)

v2.4.1.9
Stockholders' Equity - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended 0 Months Ended 1 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 12, 2013
Jul. 02, 2013
May 31, 2014
Feb. 28, 2014
Dec. 31, 2014
Nov. 30, 2013
Dec. 31, 2014
Sep. 30, 2014
Class Of Stock [Line Items]                      
Partnership distributions, amount $ 5,242us-gaap_PartnersCapitalAccountDistributions $ 29,317us-gaap_PartnersCapitalAccountDistributions $ 14,749us-gaap_PartnersCapitalAccountDistributions                
Amount distributed to existing partners 10,000samg_ExistingPartnersCapitalAccountDistributions                    
Distributions of partner incentive allocations of net income 14,266samg_DistributionsOfPartnerIncentiveAllocationsOfNetIncome 12,104samg_DistributionsOfPartnerIncentiveAllocationsOfNetIncome 8,775samg_DistributionsOfPartnerIncentiveAllocationsOfNetIncome                
Accrued partner incentive allocations 21,758us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent 17,424us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent           21,758us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent   21,758us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent  
Class A Common Stock                      
Class Of Stock [Line Items]                      
Common stock issued   5,509,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
  718,603us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
4,790,684us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
           
Exchange of Class B common stock for Class A common stock 246,000samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
2,013,000samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
                 
Common stock, par value $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
$ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
          $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
  0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
 
Class B Common Stock                      
Class Of Stock [Line Items]                      
Common stock issued 261,958us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
          261,958us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
       
Exchange of Class B common stock for Class A common stock (246,000)samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
(2,013,677)samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
                 
Common stock repurchased/surrendered and cancelled 16,530us-gaap_StockRepurchasedDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
        5,893us-gaap_StockRepurchasedDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
16,530us-gaap_StockRepurchasedDuringPeriodShares
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
       
Common stock, par value $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
$ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
      $ 17.73us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
$ 14.50us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
$ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
  0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
 
Common stock surrendered in settlement of an outstanding note receivable           4,596us-gaap_DebtConversionConvertedInstrumentSharesIssued1
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
         
Common stock surrendered in settlement of an outstanding note receivable, value           81us-gaap_DebtConversionConvertedInstrumentAmount1
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
         
Class B Common Stock | Limited Partners                      
Class Of Stock [Line Items]                      
Common stock issued               60,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_PartnerTypeOfPartnersCapitalAccountAxis
= us-gaap_LimitedPartnerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
     
Sale of common stock price per share $ 15.66us-gaap_SaleOfStockPricePerShare
/ us-gaap_PartnerTypeOfPartnersCapitalAccountAxis
= us-gaap_LimitedPartnerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
            $ 15.66us-gaap_SaleOfStockPricePerShare
/ us-gaap_PartnerTypeOfPartnersCapitalAccountAxis
= us-gaap_LimitedPartnerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
  15.66us-gaap_SaleOfStockPricePerShare
/ us-gaap_PartnerTypeOfPartnersCapitalAccountAxis
= us-gaap_LimitedPartnerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
 
Death of Chief Executive Officer | Class B Common Stock                      
Class Of Stock [Line Items]                      
Exchange of Class B common stock for Class A common stock                 2,013,677samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_ChiefExecutiveOfficerMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
   
Resale and Registration Rights Agreement | Class B Common Stock                      
Class Of Stock [Line Items]                      
Exchange of Class B common stock for Class A common stock                   110,000samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_ConversionOfStockByUniqueDescriptionAxis
= samg_ResaleAndRegistrationRightsAgreementMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
133,739samg_StockIssuedDuringPeriodSharesConversionOfConvertibleShares
/ us-gaap_ConversionOfStockByUniqueDescriptionAxis
= samg_ResaleAndRegistrationRightsAgreementMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
SLP                      
Class Of Stock [Line Items]                      
Accrued partner incentive allocations $ 18,653us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent
/ dei_LegalEntityAxis
= samg_SlpMember
$ 14,196us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent
/ dei_LegalEntityAxis
= samg_SlpMember
          $ 18,653us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent
/ dei_LegalEntityAxis
= samg_SlpMember
  18,653us-gaap_EmployeeRelatedLiabilitiesCurrentAndNoncurrent
/ dei_LegalEntityAxis
= samg_SlpMember