Derivative Instruments and Hedging Activities - Fair Value and Effect on Statement of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Cash Received (Paid) In Settlement Of Derivative Instruments Not Designated As Hedging Instruments	$ (13)	$ (569)	$ (1,009)
Asset Derivative Instruments	2	0
Liability Derivative Instruments	73	116
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	(50)	708	508
Total (Gain) Loss on Commodity Derivative Instruments	(63)	139	(501)
Crude Oil
Derivatives, Fair Value [Line Items]
Cash Received (Paid) In Settlement Of Derivative Instruments Not Designated As Hedging Instruments	(14)	(499)	(844)
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	18	582	423
Total (Gain) Loss on Commodity Derivative Instruments	4	83	(421)
Natural Gas
Derivatives, Fair Value [Line Items]
Cash Received (Paid) In Settlement Of Derivative Instruments Not Designated As Hedging Instruments	1	(70)	(147)
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	(68)	126	65
Total (Gain) Loss on Commodity Derivative Instruments	(67)	56	(82)
Natural Gas Liquids
Derivatives, Fair Value [Line Items]
Cash Received (Paid) In Settlement Of Derivative Instruments Not Designated As Hedging Instruments	0	0	(18)
Total Non-cash Portion of (Gain) Loss on Commodity Derivative Instruments	0	0	20
Total (Gain) Loss on Commodity Derivative Instruments	0	0	$ 2
Current Assets
Derivatives, Fair Value [Line Items]
Asset Derivative Instruments	2	0
Current Liabilities
Derivatives, Fair Value [Line Items]
Liability Derivative Instruments	58	102
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Asset Derivative Instruments	0	0
Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Liability Derivative Instruments	$ 15	$ 14