Equity-Based Compensation - Activity for AMTG RSUs (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013
Activity for the AMTG RSUs
Restricted stock vested, Granted to employees of the Company
AMTG Restricted Stock Unit [Member]
Activity for the AMTG RSUs
Restricted stock unvested, beginning balance	161,257
Restricted stock unvested, Granted to employees of the Company	6,250
Restricted stock unvested, Vested awards of the employees of the Company	(3,688)
Restricted stock unvested, Vested awards of the Company	(3,125)
Restricted stock unvested, ending balance	160,694
Weighted average grant date fair value, beginning balance	$ 20.28
Weighted Average Grant Date Fair Value, Granted to employees of the Company	$ 18.09
Weighted Average Grant Date Fair Value, Vested awards of the employees of the Company	$ 17.64
Weighted Average Grant Date Fair Value, Vested awards of the Company	$ 18.20
Weighted average grant date fair value, ending balance	$ 20.30
Restricted stock vested, beginning balance	12,862
Restricted stock vested, Granted to employees of the Company
Restricted stock vested, vested awards of the employees of the company	3,688
Restricted stock vested, vested awards of the company	3,125
Restricted stock vested, ending balance	19,675
Number of restricted stock outstanding, beginning balance	174,119
Number of restricted shares, Granted to employees of the Company	6,250
Number of restricted stock outstanding, ending balance	180,369