Accumulated Other Comprehensive Income AOCI by Component (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Other Comprehensive Income (Loss) Net of Tax, Period Change [Abstract]
Balance				$ 43,009
Balance		$ 43,186		43,186
Equity Method Investee [Member]
Other Comprehensive Income (Loss) Net of Tax, Period Change [Abstract]
Balance				(3)		$ (3)
Other comprehensive (loss) before reclassifications				1		(1)
Amounts reclassified from AOCI				(1)		0
Balance		(3)	$ (4)	(3)		(4)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Other Comprehensive Income (Loss) Net of Tax, Period Change [Abstract]
Balance	[1]			234		122
Other comprehensive (loss) before reclassifications	[1]			0		0
Amounts reclassified from AOCI	[2]	0	(1)	26	[1]	(1)	[1]
Balance	[1]	260	121	260		121
AOCI Attributable to Parent [Member]
Other Comprehensive Income (Loss) Net of Tax, Period Change [Abstract]
Balance				231		119
Other comprehensive (loss) before reclassifications				1		(1)
Amounts reclassified from AOCI				25		(1)
Balance		$ 257	$ 117	$ 257		$ 117

[1] Amounts are net of tax.

[2] Reclassifications out of AOCIb Three Months Ended June 30, Details about AOCI Components 2018 2017 Income Statement Line ItemAmortization of pension and retiree health and welfare benefit items Prior service credits — 1 c Total reclassifications for the period $— $1 Net of tax Six Months Ended June 30, Details about AOCI Components 2018 2017 Income Statement Line ItemAmortization of pension and retiree health and welfare benefit items Prior service credits — 1 c Reclassification due to ASU 2018-02 adoption (26) — Total reclassifications for the period $(26) $1 Net of tax