Long-Term Debt Credit Facility Available Borrowing Capacity (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014		Apr. 29, 2015		Apr. 06, 2015
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding	$ 416,500,000		$ 166,000,000
Revolving Credit Facility [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	1,500,000,000
Line of Credit Facility, Amount Outstanding	416,500,000		166,000,000
Letters of Credit Outstanding, Amount	808,000	[1]	808,000	[1]
Line of Credit Facility, Remaining Borrowing Capacity	1,082,692,000		1,333,192,000
Subsequent Event [Member] | Revolving Credit Facility [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amount Outstanding					502,500,000
Letters of Credit Outstanding, Amount					808,000	[1]
Line of Credit Facility, Remaining Borrowing Capacity					996,692,000
Borrowing Base, Line of Credit							$ 2,400,000,000

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