Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Net income	$ 38,373	$ 9,535	$ (5,165)	$ 7,125	$ 27,304	$ (3,746)	$ 11,117	$ 6,794	$ 49,868	$ 41,469	$ 62,856
Weighted average shares outstanding-basic									17,246,717	24,253,657	25,131,811
Net income per share									$ 2.89	$ 1.71	$ 2.50
Weighted average shares outstanding-diluted									17,547,061	24,505,851	25,331,420
Net income per share	$ 2.18	$ 0.54	$ (0.30)	$ 0.41	$ 1.30	$ (0.15)	$ 0.43	$ 0.26	$ 2.84	$ 1.69	$ 2.48