Earnings Per Common Share Basic and Diluted (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
Numerator — basic and diluted
Net income	$ 66,888	$ 32,214	$ (5,156)	$ 14,016	$ 7,101	$ 21,691	$ 26,547	$ 30,181	$ 107,962	$ 85,520	$ 66,053
Denominator
Weighted average number of common shares outstanding — basic (in shares)	36,906	37,431	39,555	40,527	41,201	41,239	40,820	40,506	38,697	40,943	40,846
Effect of dilutive stock options (in shares)									117	281	388
Effect of dilutive restricted shares (in shares)									378	485	495
Weighted average number of common shares outstanding — diluted (in shares)	37,643	37,746	39,555	41,245	42,018	42,065	41,599	41,148	39,192	41,709	41,729
Earnings per common share — basic (in dollars per share)	$ 1.81	$ 0.86	$ (0.13)	$ 0.35	$ 0.17	$ 0.53	$ 0.65	$ 0.75	$ 2.79	$ 2.09	$ 1.62
Earnings per common share — diluted (in dollars per share)	$ 1.78	$ 0.85	$ (0.13)	$ 0.34	$ 0.17	$ 0.52	$ 0.64	$ 0.73	$ 2.75	$ 2.05	$ 1.58
Stock Options And Restricted Shares
Denominator
Antidilutive stock options and restricted shares (in shares)									1,561	1,404	1,734