Earnings Per Share Basic and Diluted (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basic:
Net income	$ 23,659	$ 51,821	$ 105,180	$ 148,809
Net income attributable to participating securities	(540)	(976)	(2,404)	(2,807)
Net income attributable to common stock - basic	23,119	50,845	102,776	146,002
Diluted:
Net income	23,659	51,821	105,180	148,809
Net income attributable to participating securities	(540)	(975)	(2,402)	(2,805)
Net income attributable to common stock - diluted	$ 23,119	$ 50,846	$ 102,778	$ 146,004
Weighted average shares (denominator):
Weighted average shares - basic	48,342	48,029	48,300	47,963
Diluted effect of stock options	42	42	43	43
Weighted average shares - diluted	48,384	48,071	48,343	48,006
Basic income per common share	$ 0.48	$ 1.06	$ 2.13	$ 3.04
Diluted income per common share	$ 0.48	$ 1.06	$ 2.13	$ 3.04