Equity-Based Compensation - Activity for AMTG RSUs (Detail) (AMTG Restricted Stock Unit, USD $)	3 Months Ended
	Mar. 31, 2014
AMTG Restricted Stock Unit
Activity for the AMTG RSUs
Restricted stock unvested, beginning balance	127,237
Restricted stock unvested, vested	(989)
Restricted stock unvested, vested to the Company	(1,562)
Restricted stock unvested, ending balance	124,686
Weighted average grant date fair value, beginning balance	$ 19.28
Weighted Average Grant Date Fair Value, Vested awards of the employees of the Company	$ 16.60
Weighted Average Grant Date Fair Value, Vested awards of the Company	$ 18.20
Weighted average grant date fair value, ending balance	$ 19.31
Restricted stock vested, beginning balance	70,371
Restricted stock vested, vested awards of the employees of the company	989
Restricted stock vested, vested awards of the company	1,562
Share Based Compensation Restricted Stock Vested Ending Balance	72,922
Number of restricted stock outstanding, beginning balance	197,608
Number of restricted stock outstanding, ending balance	197,608