Financing Agreements - Additional Information (Details) - USD ($)			1 Months Ended	3 Months Ended			6 Months Ended
	Mar. 16, 2015	Jan. 08, 2015	Dec. 31, 2014	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Debt Instrument [Line Items]
Loss on extinguishment of debt							$ (2,000,000)
Revolving Facility [Member]
Debt Instrument [Line Items]
Current aggregate facility				$ 500,000,000			$ 500,000,000
Line of credit facility, unused capacity, benchmark							50.00%
Line of credit facility, maximum letters of credit				300,000,000			$ 300,000,000
Fronting fee rate							0.125%
Line of credit facility, amount outstanding				0			$ 0
Utilized letters of credit				39,000,000			39,000,000
Borrowing base collateral				431,000,000			431,000,000
Available borrowing capacity				$ 392,000,000			$ 392,000,000
Revolving Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commitment fee rate							0.25%
Revolving Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Commitment fee rate							0.375%
Senior Notes Due December 2024 [Member]
Debt Instrument [Line Items]
Senior notes issued			$ 425,000,000
Net proceeds of the offering			418,000,000
Financing costs			7,000,000
Senior Notes Due February 2019 [Member]
Debt Instrument [Line Items]
Debt redeemed	$ 15,000,000	$ 40,000,000	$ 345,000,000
Weighted average debt redemption price	103.25%	103.00%	104.116%
Loss on extinguishment of debt			$ 19,000,000		$ 2,000,000
Write-off of deferred financing cost			$ 4,000,000