Annual report pursuant to Section 13 and 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 $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Jul. 12, 2013
May 31, 2014
Feb. 28, 2014
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 7,768,010us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		7,522,974us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember 		5,509,297us-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 		  $ 17.73us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		$ 14.50us-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 4,520,413us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		4,464,617us-gaap_CommonStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember 		     
Deferred equity units exercisable 52,188us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassBMember