Earnings Per Share - Computation of Basic and Diluted (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Income (Loss)	$ 58,976	$ 15,661	$ 63,730	$ 40,155
Denominator:
Denominator for basic earnings per share - Weighted-average shares	16,484,957	16,106,054	16,342,289	15,981,337
Effect of dilutive stock options (in shares)	539,000	828,000	561,000	733,000
Denominator for diluted earnings per share - Adjusted weighted-average shares and assumed conversions	17,024,137	16,934,423	16,903,224	16,713,837
Basic income (loss) per share (in dollars per share)	$ 3.58	$ 0.97	$ 3.90	$ 2.52
Diluted income (loss) per share (in dollars per share)	$ 3.46	$ 0.93	$ 3.77	$ 2.40