Employment Benefit Plans AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Defined Benefit Plan, Change in Accumulated Other Comprehensive Income Loss [Roll Forward]
Beginning balance	$ 207	$ 102	$ 34
Amortization of net loss	0	1	2
Amortization of prior service credits	0	(1)	(1)
Actuarial (loss) gain	164	106	68
Settlements	0	(1)	(1)
Ending balance	371	207	102
Expected Amortization of Prior Service Credit, Next Fiscal Year				$ 1
Expected Amortization of Loss, Next Fiscal Year				1
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net gain (loss)				370	$ 206
Prior Service Credits				4	4
Settlements				(3)	(3)
Pre-tax amount recognized in AOCI at December 31,	207	102	34	371	207
After-tax amount recognized in AOCI at December 31,				251	128
Retiree Health and Welfare Benefits [Member]
Defined Benefit Plan, Change in Accumulated Other Comprehensive Income Loss [Roll Forward]
Beginning balance	(9)	(21)	(34)
Amortization of net loss	0	1	2
Amortization of prior service credits	(2)	(2)	(2)
Actuarial (loss) gain	(14)	13	13
Prior Service credit	1	0	0
Ending balance	(24)	(9)	(21)
Expected Amortization of Prior Service Credit, Next Fiscal Year				2
Expected Amortization of Loss, Next Fiscal Year				1
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net gain (loss)				(28)	(14)
Prior Service Credits				4	5
Settlements				0	0
Pre-tax amount recognized in AOCI at December 31,	$ (9)	$ (21)	$ (34)	(24)	(9)
After-tax amount recognized in AOCI at December 31,				$ (17)	$ (6)