Earnings Per Share - Computation of Basic Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Calculation Of Numerator And Denominator In Earnings Per Share [Line Items]
Allocation of net income (loss) to common stock	$ 20,558	$ 15,379	$ 70,009	$ 53,141
Earnings (loss) per share:
Common stock-basic (usd per share)	$ 0.34	$ 0.23	$ 1.14	$ 0.81
Common Stock [Member]
Calculation Of Numerator And Denominator In Earnings Per Share [Line Items]
Allocation of net income (loss) to common stock	$ 20,415	$ 15,319	$ 69,536	$ 52,936
Weighted average number of common shares:
Weighted average number (shares)	60,290,842	66,087,184	61,112,263	65,755,911
Unvested Participating Shares [Member]
Calculation Of Numerator And Denominator In Earnings Per Share [Line Items]
Allocation of net income (loss) to common stock	$ 143	$ 60	$ 473	$ 205
Weighted average number of common shares:
Weighted average number (shares)	422,725	259,525	415,472	254,719