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
	Mar. 31, 2013	Mar. 31, 2012
Basic:
Net income	$ 40,758	$ 50,974
Net income attributable to participating securities	(770)	(877)
Net income attributable to common stock-basic	39,988	50,097
Diluted:
Net income	40,758	50,974
Net income attributable to participating securities	(770)	(876)
Net income attributable to common stock-diluted	$ 39,988	$ 50,098
Weighted average shares (denominator):
Weighted average shares-basic	48,619	48,254
Diluted effect of stock options	38	45
Weighted average shares-diluted	48,657	48,299
Basic income per common share	$ 0.82	$ 1.04
Diluted income per common share	$ 0.82	$ 1.04