Derivatives - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Net loss on commodity derivatives	$ (91)	$ (102)
Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Net loss on commodity derivatives	(113)	(43)
Net gain (loss) on settlements, net	$ 22	$ (59)