Summary of Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current Income Tax Expense:
Federal	$ 116,234
State	10,819	9,797	8,932
Current Income Tax Expense (Benefit), Total	127,053	9,797	8,932
Deferred Income Tax (Benefit) Expense:
Federal	(23,035)	62,015	64,679
State	(206)	(2,202)	(1,201)
Deferred Income Tax Expense (Benefit), Total	(23,241)	59,813	63,478
INCOME TAXES	$ 103,812	$ 69,610	$ 72,410