Reconciliation of 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 [Abstract]
Net income for basic and diluted earnings per share	$ 99,943	$ 15,334	$ 9,542	$ 10,334	$ 18,244	$ 17,924	$ 20,671	$ 17,965	$ 135,153	$ 74,805	$ 70,949
Weighted average shares outstanding - basic									33,220	33,841	35,876
Dilutive effect of stock options and restricted stock									130	108	92
Weighted average shares outstanding - diluted									33,350	33,949	35,968
Earnings per share - basic	$ 3.01	$ 0.46	$ 0.29	$ 0.31	$ 0.55	$ 0.54	$ 0.61	$ 0.51	$ 4.07	$ 2.21	$ 1.98
Earnings per share - diluted	$ 2.99	$ 0.46	$ 0.29	$ 0.31	$ 0.55	$ 0.54	$ 0.61	$ 0.51	$ 4.05	$ 2.20	$ 1.97