Debt - Revolving Credit Facility Narrative (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended	27 Months Ended
	Dec. 31, 2017	Sep. 30, 2015	Dec. 31, 2018	Dec. 31, 2017
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Borrowing Capacity, Description		The amended credit facility provides that, while the obligations are required to be secured, (i) we have the right to incur junior lien indebtedness of up to $2.0 billion; (ii) our use of the facility will be subject to a borrowing base; (iii) the rate of interest on outstanding loans, as well as fees on undrawn commitments, will vary based on the percentage of the borrowing base used, rather than on our credit ratings; (iv) the total leverage ratio covenant will be suspended; and (v) the credit facility will be subject to a first lien secured leverage ratio and an interest rate coverage ratio (as described below). The amendment sets the borrowing base at $4.0 billion. The total commitments under the credit facility remain at $4.0 billion, subject to reduction in connection with issuances of junior lien indebtedness by us after April 15, 2016, the date of the first borrowing base redetermination. No adjustment to the total commitment will occur for any junior lien indebtedness issuance that occurs before April 15, 2016.
Line of Credit Facility, Covenant Terms		a requirement that we maintain, as of the last day of each fiscal quarter, a net debt to capitalization ratio (as defined in the amended credit agreement) that does not exceed 65%. While it is required to be secured by a portion of our oil and natural gas properties, the amended credit facility requires us to maintain, as of the last day of each fiscal quarter (i) a first lien secured leverage ratio (as defined in the amended credit agreement) of 3.5 to 1.0 through 2017 and 3.0 to 1.0 thereafter, and (ii) an interest rate coverage ratio (as defined in the amended credit agreement) of 1.1 to 1.0 through the first quarter of 2017, increasing to 1.25 to 1.0 by the end of 2017.
Revolving Credit Facility [Member]
Long-Term Debt Instrument [Line Items]
Borrowing capacity		$ 4,000
Long-term Debt, Gross		0
Letters of Credit Outstanding, Amount		$ 12
Interest Rate In Addition To Federal Funds Rate		0.50%
Loans Receivable, Basis Spread on Variable Rate		1.00%
Line of Credit Facility, Guarantee Event One		cross-payment default and cross acceleration with respect to indebtedness in an aggregate principal amount of $125 million or more
Line of Credit Facility, Guarantee Event Two		bankruptcy; judgments involving liability of $125 million or more that are not paid
Line of Credit Facility, Guarantee Event Three		ERISA events
Credit Facility Default Value, Cross Payment Default and Cross Acceleration With Respect to Indebtedness Aggregate Minimum Principal Amount		$ 125
Credit Facility Default Value, Bankruptcy - Judgments Involving Liability, Minimum Principal Amount		$ 125
Revolving Credit Facility [Member] | Alternative Base Rate (ABR) [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		1.00%
Revolving Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		2.00%
Revolving Credit Facility [Member] | Maximum [Member]
Long-Term Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital		0.65
Revolving Credit Facility [Member] | Junior Lien [Member]
Long-Term Debt Instrument [Line Items]
Borrowing capacity		$ 2,000
Revolving Credit Facility [Member] | Senior Lien [Member]
Long-Term Debt Instrument [Line Items]
Borrowing capacity		$ 4,000
Revolving Credit Facility [Member] | First Lien Secured Leverage Ratio [Member] | Scenario, Forecast [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Covenant Description			3.0 to 1	3.5 to 1
Revolving Credit Facility [Member] | Interest Rate Coverage Ratio [Member] | Scenario, Forecast [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Covenant Description	1.25 to 1.0			1.1 to 1.0