Basis of Presentation and Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions				3 Months Ended		12 Months Ended
	May. 02, 2016	Apr. 08, 2016	Mar. 31, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Aug. 31, 2017	May. 31, 2017	Jan. 31, 2017	Dec. 31, 2014
Summary of Significant Accounting Policies [Table] [Line Items]
Impairment of oil and natural gas properties				$ 853	$ 4,976
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest				(921)	(5,092)
Cash and cash equivalents ($1 and $1 attributable to our VIE)			$ 16	16	$ 2,907		$ 825					$ 4,108
Working Capital (Deficit)			(1,341)	(1,341)			(1,205)
Debt Instrument, Face Amount			9,425	9,425			9,706
Long-term Debt, Maturities, Repayments of Principal in Year Three			878	878
Payments to Acquire Productive Assets							3,600
Long-term Debt, Gross			10,405	10,405			10,692
Senior Notes [Member] | 6.25% Euro-Denominated Senior Notes Due 2017 [ Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount			$ 344	$ 344			329
Debt Instrument, Interest Rate, Stated Percentage			6.25%	6.25%
Long-term Debt, Gross			$ 344	$ 344			329
Senior Notes [Member] | 3.25% Senior Notes due 2016 [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount			$ 0	$ 0			381
Debt Instrument, Interest Rate, Stated Percentage			3.25%	3.25%
Long-term Debt, Gross			$ 0	$ 0			381
Senior Notes [Member] | 2.5% Contingent Convertible Senior Notes due 2037 [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount			$ 902	$ 902			1,110
Debt Instrument, Interest Rate, Stated Percentage			2.50%	2.50%
Long-term Debt, Gross			$ 846	$ 846			1,027
Senior Notes [Member] | 6.5% Senior Notes Due 2017 [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount			$ 379	$ 379			453
Debt Instrument, Interest Rate, Stated Percentage			6.50%	6.50%
Long-term Debt, Gross			$ 379	$ 379			$ 453
Senior Notes [Member] | 6.75% Senior Notes Due 2019 [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			6.775%	6.775%
Revolving Credit Facility [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Borrowing capacity			$ 4,000	$ 4,000
Long-term Debt, Gross			$ 367	$ 367
Standard & Poor's, BB- Rating [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Credit Rating				BB-
Standard & Poor's, CC Rating [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Credit Rating			CC
Moody's, Ba3 Rating [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Credit Rating				Ba3
Moody's, Caa3 Rating [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Credit Rating			“Caa3
Subsequent Event [Member]
Summary of Significant Accounting Policies [Table] [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
Collateral Posted	$ 247
Line of Credit Facility, Collateral	We have posted the required collateral, primarily in the form of letters of credit and cash, or are otherwise complying with these contractual requests for collateral.
Subsequent Event [Member] | Senior Notes [Member] | 6.5% Senior Notes Due 2017 [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount									$ 379
Subsequent Event [Member] | Revolving Credit Facility [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Percentage of Company's Assets, Including Mortgages, Used as Security Interests		90.00%
Borrowing capacity		$ 4,000
Cash Collateral for Borrowed Securities	$ 281
Scenario, Forecast [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Two								$ 1,625
Scenario, Forecast [Member] | Senior Notes [Member] | 6.25% Euro-Denominated Senior Notes Due 2017 [ Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount											$ 344
Scenario, Forecast [Member] | Senior Notes [Member] | 2.5% Contingent Convertible Senior Notes due 2037 [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Debt Instrument, Face Amount										$ 902
Scenario, Forecast [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Payments to Acquire Productive Assets						$ 1,300
Scenario, Forecast [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Payments to Acquire Productive Assets						$ 1,800
Scenario, Forecast [Member] | Subsequent Event [Member] | Revolving Credit Facility [Member]
Summary of Significant Accounting Policies [Table] [Line Items]
Cash Collateral for Borrowed Securities	$ 696