Income Taxes - Components of Deferred Tax Assets and Liabilities (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Components Of Deferred Tax Assets And Liabilities [Line Items]
Income tax receivable	$ 9,700,000	$ 0
Provision for income taxes	448,830,000	415,811,000	258,373,000
Increase (decrease) in deferred income tax liabilities	(442,621,000)	(405,294,000)	(245,203,000)
Stock Compensation Plan [Member]
Components Of Deferred Tax Assets And Liabilities [Line Items]
Increase (decrease) in deferred income tax liabilities		(15,600,000)
Wheatland Oil Inc. [Member]
Components Of Deferred Tax Assets And Liabilities [Line Items]
Increase (decrease) in deferred income tax liabilities		$ 56,600,000