Long-Term Debt (Details) (USD $)	9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Oct. 23, 2013		Sep. 06, 2013	May 20, 2013	Apr. 12, 2013	Dec. 31, 2012
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	1,775,000,000	1,900,000,000
Reduction in Borrowing Base as a Percent of Senior Debt Prinicipal	25.00%
Line of Credit, Covenant Compliance, Maximum Annual Dividend Payment	50,000,000
Revolving credit facility	28,000,000			59,000,000					340,000,000
Letters of Credit Outstanding, Amount	800,000	[1]		800,000	[1]				800,000	[1]
Line of Credit Facility, Remaining Borrowing Capacity	1,271,200,000			1,240,200,000					659,200,000
Deferred Finance Costs, Gross	3,400,000
Proceeds from Debt Issuance, Net of Issuance Costs, 5% Senior Notes	490,274,000		0
5.0% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Interest rate on notes (as a percent)	5.00%						5.00%		5.00%
Deferred Finance Costs, Gross							9,700,000
Debt Instrument, Maximum Early Redemption, Percent Of Principal	35.00%
Debt Instrument Future Redemption Price As Percentageof Principal First Through Third Year	105.00%
Debt Instrument, Call Feature	at a redemption price equal to 100 percent of the principal amount of the 2024 Notes to be redeemed, plus a specified make-whole premium and accrued and unpaid interest to the applicable redemption date.
Debt Instrument, Covenant Compliance, Dividends Excluded From Computation	6,500,000
Debt Instrument, Face Amount							$ 500,000,000
2018 [Member] | 5.0% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	102.50%
2019 [Member] | 5.0% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	101.667%
2020 [Member] | 5.0% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.833%
2021 and Thereafter [Member] | 5.0% Senior Notes Due 2024 [Member]
Debt Instrument [Line Items]
Early Debt Redemption, Redemption Price as a Percent of Face Amount	100.00%

[1] (1) Letters of credit reduce the available borrowing capacity under the credit facility on a dollar-for-dollar basis.