Components of Provision for Income Taxes (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current provision:
Federal									$ 86,220	$ 47,794	$ 25,300
State									25,505	15,042	7,813
Deferred expense (benefit):
Federal									5,756	(1,157)	1,445
State									1,606	(277)	649
Income tax expense	$ 26,284	$ 26,770	$ 43,263	$ 22,770	$ 11,005	$ 24,996	$ 13,819	$ 11,582	$ 119,087	$ 61,402	$ 35,207