Fair Value Measurement - FV of Commodity Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Derivative Assets [Abstract]
Assets - gross amounts recognized	$ 311	$ 312
Assets - gross amounts offset	186	221
Assets - net amounts presented	125	91
Assets - cash collateral	0	0
Assets - amounts without right of setoff	4	5
Commodity derivatives, assets	121	86
Offsetting Derivative Liabilities [Abstract]
Liabilities - gross amounts recognized	310	334
Liabilities - gross amounts offset	186	221
Liabilities - net amounts presented	124	113
Liabilities - cash collateral	7	12
Liabilities - amounts without right of setoff	5	12
Commodity derivatives	$ 112	$ 89