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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share Disclosure [Line Items]
Net income (loss)	$ 28,044	$ 12,915	$ 14,899	$ 7,904	$ 10,088	$ 13,898	$ 9,504	$ (7,893)	$ 63,762	$ 25,597	$ 69,578
Basic weighted average shares									14,052,311	14,980,429	16,064,770
Effect of common stock equivalents:
Assumed exercise of stock options and vesting of stock grants									275,509	202,856	350,496
Diluted weighted average shares									14,327,820	15,183,285	16,415,266
Net income (loss) per common share:
Basic	$ 2.00	$ 0.92	$ 1.06	$ 0.57	$ 0.71	$ 0.94	$ 0.62	$ (0.50)	$ 4.54	$ 1.71	$ 4.33
Diluted	$ 1.96	$ 0.90	$ 1.05	$ 0.56	$ 0.70	$ 0.92	$ 0.60	$ (0.50)	$ 4.45	$ 1.69	$ 4.24