Derivatives - Narrative (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on commodity derivatives		$ (14)	$ (36)	$ (66)
Net settlements of derivative instruments		(281)	45	44
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		$ 0	47	0
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax			46
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash Proceeds Received from Derivatives	$ 54		$ 54
Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) on commodity derivatives				(110)
Derivative Instrument Not Designated As Hedging Instruments, Gain (Loss) On Settlements, Net				$ 44
Unsecured Debt [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Debt Instrument, Face Amount	$ 1,000