Long-Term Debt (Details) (USD $)			3 Months Ended			3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Line of Credit [Member]	Mar. 31, 2014 5.0% Senior Notes Due 2024 [Member]	May 20, 2013 5.0% Senior Notes Due 2024 [Member]	Mar. 31, 2014 Revolving Credit Facility [Member]	Mar. 31, 2014 Senior Notes [Member]	Dec. 31, 2013 Senior Notes [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 2,500,000,000
Line of Credit Facility, Current Borrowing Capacity			1,300,000,000
Borrowing Base, Line of Credit			2,200,000,000
Percentage of Proved Oil and Gas Properties Secured for Credit Facility Borrowing						75.00%
Line of Credit, Covenant Compliance, Maximum Annual Dividend Payment			50,000,000
Debt Instrument, Covenant Compliance, Dividends Excluded From Computation				6,500,000
Interest rate on notes (as a percent)					5.00%
Debt Instrument, Face Amount					500,000,000
6.625% Senior Notes, Due 2019							350,000,000	350,000,000
6.50% Senior Notes, Due 2021							350,000,000	350,000,000
6.50% Senior Notes, Due 2023							400,000,000	400,000,000
5% Senior Notes, Due 2024							500,000,000	500,000,000
Senior Notes	$ 1,600,000,000	$ 1,600,000,000					$ 1,600,000,000	$ 1,600,000,000