Reconciliation of Basic and Diluted Earnings Per Share Computations (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share Disclosure [Line Items]
Net income (loss)	$ 10,088	$ 13,898	$ 9,504	$ (7,893)	$ 17,338	$ 16,225	$ 18,970	$ 17,045	$ 25,597	$ 69,578	$ 63,158
Basic weighted average shares									14,980,429	16,064,770	16,935,488
Effect of common stock equivalents:
Assumed exercise of stock options and vesting of stock grants									202,856	350,496	386,532
Diluted weighted average shares									15,183,285	16,415,266	17,322,020
Net income (loss) per common share:
Basic	$ 0.71	$ 0.94	$ 0.62	$ (0.50)	$ 1.10	$ 1.03	$ 1.18	$ 1.02	$ 1.71	$ 4.33	$ 3.73
Diluted	$ 0.70	$ 0.92	$ 0.60	$ (0.50)	$ 1.07	$ 1.00	$ 1.16	$ 1.00	$ 1.69	$ 4.24	$ 3.65