Earnings Per Share - Computation of Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Calculation Of Numerator And Denominator In Earnings Per Share [Line Items]
Net income									$ 156,963	$ 94,760	$ 93,927
Allocation of net income to common stockholders: common stock	$ 51,111	$ 30,905	$ 32,828	$ 41,151	$ 16,965	$ 22,306	$ 30,131	$ 24,517	155,995	93,919	93,287
Allocation of net income to common stockholders: Unvested participating shares									968	841	640
Net income (loss) available to common shareholders									156,963	94,760	93,927
Less adjusted earnings allocated to unvested participating shares									(947)	(822)	(624)
Less adjusted earnings allocated to common stock									$ 156,016	$ 93,938	$ 93,303
Weighted average number of common shares:
Common stock-basic (in shares)									58,873,196	59,229,069	60,835,574
Common stock-diluted (in shares)									60,253,691	60,594,895	62,360,778
Earnings (loss) per common share:
Common stock-basic (in dollars per share)	$ 0.88	$ 0.53	$ 0.56	$ 0.69	$ 0.29	$ 0.38	$ 0.51	$ 0.41	$ 2.65	$ 1.59	$ 1.53
Common stock-diluted (in dollars per share)	$ 0.86	$ 0.51	$ 0.54	$ 0.68	$ 0.28	$ 0.37	$ 0.50	$ 0.40	$ 2.59	$ 1.55	$ 1.50
Common Stock
Weighted average number of common shares:
Common stock-basic (in shares)									58,873,196	59,229,069	60,835,574
Weighted average number of common shares - effect of dilutive securities (in shares)									1,380,495	1,365,826	1,525,204
Common stock-diluted (in shares)									60,253,691	60,594,895	62,360,778
Earnings (loss) per common share:
Common stock-basic (in dollars per share)									$ 2.65	$ 1.59	$ 1.53
Common stock-diluted (in dollars per share)									$ 2.59	$ 1.55	$ 1.50
Common Stock | Unvested Participating Shares
Weighted average number of common shares:
Common stock-basic (in shares)									366,029	531,364	417,285