Employment Benefit Plans Change in Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of period	$ 2,247	$ 2,320
Service cost	42	46	$ 48
Interest cost	88	95	90
Actuarial loss (gain)	166	(55)
Benefits Paid	(144)	(143)
Settlements	(12)	(16)
Projected benefit obligation at end of period	2,387	2,247	2,320
Component representing future salary increases	(135)	(102)
Accumulated benefit obligation at end of period	2,252	2,145
Retiree Health and Welfare Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of period	237	258
Service cost	1	1	1
Interest cost	9	10	10
Plan participants' contributions	4	5
Actuarial loss (gain)	14	(13)
Benefits Paid	(23)	(24)
Plan amendment	(1)	0
Projected benefit obligation at end of period	$ 241	$ 237	$ 258