Long-Term Debt - Credit Facility (Details)			6 Months Ended
	Apr. 02, 2014 USD ($)	Mar. 10, 2011 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Long-term debt, gross			$ 985,000,000	$ 959,000,000
Credit Facility due 2016
Line of Credit Facility [Line Items]
Expiration period		5 years
Current borrowing capacity		$ 1,000,000,000
Credit facility due 2019
Line of Credit Facility [Line Items]
Expiration period	5 years
Maximum borrowing capacity	$ 1,500,000,000		700,000,000
Minimum percent of total value securing credit agreement	80.00%
Purchase price of properties as a percentage of borrowing base required for potential re-determination of borrowing base, minimum	10.00%
Minimum percent of outstanding principal amount required for changes to credit agreement	66.6667%
Long-term debt, gross			$ 135,000,000	$ 109,000,000
Interest rate at period end			1.69%
Remaining borrowing capacity			$ 564,900,000
Interest Paid			$ 2,400,000
Credit facility due 2019 | Maximum
Line of Credit Facility [Line Items]
Ratio of indebtedness to EBITDA			2.5
Credit facility due 2019 | Minimum
Line of Credit Facility [Line Items]
Ratio of indebtedness to EBITDA				2.5
Ratio of unused amount of total commitments to consolidated current liabilities			1.0
Credit facility due 2019 | one-, two-, three- or six-month LIBOR [Member] | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.50%
Credit facility due 2019 | one-, two-, three- or six-month LIBOR [Member] | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Credit facility due 2019 | Alternate Base Rate, Federal Funds [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Credit facility due 2019 | Alternate Base Rate, one-month LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Credit facility due 2019 | Standard Alternate Base Rate [Member] | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Credit facility due 2019 | Standard Alternate Base Rate [Member] | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 2,000,000