Debt - Revolving Credit Facility Narrative (Details) - USD ($) $ in Millions					3 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Apr. 08, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Long-Term Debt Instrument [Line Items]
Long-term Debt, Gross									$ 10,405		$ 10,692
Debt Instrument, Face Amount									9,425		$ 9,706
Interest expense on senior notes									115	$ 171
Subsequent Event [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Borrowing Capacity, Description				(i) the suspension or modification of certain financial covenants and (ii) the granting of liens and security interests on substantially all of our assets, including mortgages encumbering 90% of our proved oil and gas properties that constitute borrowing base properties, all hedge contracts and personal property, subject to certain agreed upon carve outs
Revolving Credit Facility [Member]
Long-Term Debt Instrument [Line Items]
Long-term Debt, Gross									367
Letters of Credit Outstanding, Amount									619
Borrowing capacity									4,000
Revolving Credit Facility [Member] | Scenario, Forecast [Member]
Long-Term Debt Instrument [Line Items]
Interest Coverage Ratio	1.25 to 1.0							1.1 to 1.0
Amended Interest Coverage Ratio		1.25 to 1.0	1.1 to 1.0		1.25 to 1.0	1.2 to 1.0	0.70 to 1.0	0.65 to 1.0
Minimum liquidity requirement when covenant ratio is above 1.1 to 1.0						$ 500
Minimum liquidity requirement when covenant ratio is below 1.1 to 1.0			$ 750
Revolving Credit Facility [Member] | Subsequent Event [Member]
Long-Term Debt Instrument [Line Items]
Borrowing capacity				$ 4,000
Revolving Credit Facility [Member] | Subsequent Event [Member] | Scenario, Forecast [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Covenant Terms				The amendment reduces the interest coverage ratio from 1.1 to 1.0 to 0.65 to 1.0 through the first quarter of 2017, after which it will increase to 0.70 to 1.0 through the second quarter of 2017, 1.2 to 1.0 through the third quarter of 2017 and 1.25 to 1.0 thereafter. The amendment also includes a collateral value coverage test whereby if the collateral value coverage ratio, tested as of December 31, 2016, falls below 1.1 to 1.0, the $500 million minimum liquidity covenant increases to $750 million, and if the collateral value coverage ratio, tested as of March 31, 2017, falls below 1.25 to 1.0, our borrowing ability will be reduced in order to satisfy such ratio.
Revolving Credit Facility [Member] | Subsequent Event [Member] | First Lien [Member]
Long-Term Debt Instrument [Line Items]
Borrowing capacity				$ 2,500
6.75% Senior Notes Due 2019 [Member] | Senior Notes [Member]
Long-Term Debt Instrument [Line Items]
Supersedeas Bond									$ 461