Reconciliation of Basic and Diluted Earnings per Share Computations (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
Earning Per Share Basic And Diluted [Abstract]
Net Income	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Basic weighted average shares									29,441	29,074	28,785
Effect of Common Stock Equivalents:
Assumed Exercise of Stock Options and Vesting of Stock Grants									630	958	866
Diluted Weighted Average Shares									30,071	30,032	29,651
Net Income Per Common Share:
Basic	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 0.74	$ 0.76	$ 0.56	$ 0.79	$ 1.38	$ 2.85	$ 2.82
Diluted	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 0.71	$ 0.73	$ 0.54	$ 0.77	$ 1.35	$ 2.75	$ 2.73