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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earning Per Share Basic And Diluted [Abstract]
Net Income	$ (33,368)	$ 4,625	$ 32,104	$ 30,878	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 34,239	$ 40,494	$ 82,726
Basic weighted average shares									29,720	29,441	29,074
Effect of Common Stock Equivalents:
Assumed Vesting of Stock Grants									473	630	958
Diluted Weighted Average Shares									30,193	30,071	30,032
Net Income Per Common Share:
Basic	$ (1.12)	$ 0.16	$ 1.08	$ 1.04	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 1.15	$ 1.38	$ 2.85
Diluted	$ (1.12)	$ 0.15	$ 1.07	$ 1.02	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 1.13	$ 1.35	$ 2.75