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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic:
Net income	$ 36,102	$ 23,659	$ 115,882	$ 105,180
Net income attributable to participating securities	(924)	(564)	(2,728)	(2,283)
Net income attributable to common stock-basic	35,178	23,095	113,154	102,897
Diluted:
Net income	36,102	23,659	115,882	105,180
Net income attributable to participating securities	(923)	(563)	(2,726)	(2,281)
Net income attributable to common stock-diluted	$ 35,179	$ 23,096	$ 113,156	$ 102,899
Weighted average shares (denominator):
Weighted average shares-basic	48,732	48,342	48,680	48,300
Dilutive effect of stock options	44	42	40	43
Weighted average shares-diluted	48,776	48,384	48,720	48,343
Basic earnings per share	$ 0.72	$ 0.48	$ 2.32	$ 2.13
Diluted earnings per share	$ 0.72	$ 0.48	$ 2.32	$ 2.13