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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basic:
Net income	$ 39,022	$ 30,547	$ 79,780	$ 81,521
Net income attributable to participating securities	(1,015)	(734)	(1,794)	(1,681)
Net income attributable to common stock-basic	38,007	29,813	77,986	79,840
Diluted:
Net income	39,022	30,547	79,780	81,521
Net income attributable to participating securities	(1,015)	(733)	(1,793)	(1,680)
Net income attributable to common stock-diluted	$ 38,007	$ 29,814	$ 77,987	$ 79,841
Weighted average shares (denominator):
Weighted average shares-basic	48,687	48,303	48,653	48,279
Diluted effect of stock options	38	41	38	43
Weighted average shares-diluted	48,725	48,344	48,691	48,322
Basic earnings per share	$ 0.78	$ 0.62	$ 1.60	$ 1.65
Diluted earnings per share	$ 0.78	$ 0.62	$ 1.60	$ 1.65