Earnings per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 10,264	$ 11,196	$ 11,778	$ 10,856	$ 6,918	$ 58	$ 5,240	$ 7,289	$ 44,094	$ 19,505	$ 27,268
Net income allocated to unvested restricted stock									(409)	(167)	(204)
Net income allocated to common shares									$ 43,685	$ 19,338	$ 27,064
Denominator for basic earnings per share—weighted-average shares outstanding	27,254	26,437	26,380	26,345	26,300	26,273	19,434	19,402	26,606	22,871	19,491
Effect of dilutive securities—employee stock options									102	60	31
Denominator for diluted earnings per share—adjusted weighted-average shares outstanding	27,356	26,542	26,477	26,448	26,436	26,340	19,469	19,433	26,708	22,931	19,522
Basic earnings per share	$ 0.37	$ 0.42	$ 0.44	$ 0.41	$ 0.27	$ 0.00	$ 0.27	$ 0.37	$ 1.64	$ 0.85	$ 1.39
Diluted earnings per share	$ 0.37	$ 0.42	$ 0.44	$ 0.41	$ 0.27	$ 0.00	$ 0.27	$ 0.37	$ 1.64	$ 0.84	$ 1.39
Average anti-dilutive options excluded from computation of diluted earnings per share									169	280	496