Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail)

v2.4.1.9
Stockholders' Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Class Of Stock [Line Items]        
Preferred stock, par value $ 0.01us-gaap_PreferredStockParOrStatedValuePerShare $ 0.01us-gaap_PreferredStockParOrStatedValuePerShare    
Class A Common Stock        
Class Of Stock [Line Items]        
Common stock, par value $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		$ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		   
Common stock, voting rights 1 vote per share      
Common stockholders rights percentage 100.00%samg_CommonStockHoldingRightsPercentage
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		     
Common stock, shares outstanding 7,786,010us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		7,768,010us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		7,523,000us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		7,523,000us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
Class B Common Stock        
Class Of Stock [Line Items]        
Common stock, par value $ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		$ 0.01us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		   
Common stock, voting rights 1 vote per share      
Class of share exchangeable to another class one share of Class A common stock      
Common stock, shares outstanding 4,629,029us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		4,520,413us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		4,710,000us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		4,465,000us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember
Deferred equity units exercisable 4,911us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember