Calculation of Basic and Diluted Weighted Average Shares Outstanding and Earnings Per Share (Detail) (USD $) In Thousands, except Per 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
Basic:
Net income									$ 149,426	$ 194,332	$ 96,429
Net income attributable to participating securities									(2,984)	(3,670)	(1,559)
Net income attributable to common stock - basic									146,442	190,662	94,870
Diluted:
Net income									149,426	194,332	96,429
Net income attributable to participating securities									(2,982)	(3,667)	(1,558)
Net income attributable to common stock - diluted									$ 146,444	$ 190,665	$ 94,871
Weighted average shares (denominator):
Weighted average shares - basic									48,319	47,988	47,681
Diluted effect of stock options									42	42	25
Weighted average shares - diluted									48,361	48,030	47,706
Basic income per common share	$ 0.89	$ 0.48	$ 0.62	$ 1.03	$ 0.93	$ 1.06	$ 1.17	$ 0.81	$ 3.03	$ 3.97	$ 1.99
Diluted income per common share	$ 0.89	$ 0.48	$ 0.62	$ 1.03	$ 0.93	$ 1.06	$ 1.17	$ 0.81	$ 3.03	$ 3.97	$ 1.99