Employee Pension and Other Postretirement Benefits - Changes in Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Accumulated benefit obligation	$ 2,850	$ 2,679
Change in benefit obligation
Benefit obligation, beginning of year	2,816	2,817
Service cost	91	96	$ 81
Interest cost	103	110	105
Plan participants' contributions	0	0
Actuarial loss/(gain)	117	(114)
Effect of plan amendments	(76)	0
Medicare Part D subsidy	0	0
Benefits paid	(109)	(93)
Benefit obligation, end of year	2,942	2,816	2,817
Fair value of plan assets, end of year	3,254	2,992	2,897
Over (Under) funded status	312	176
Other Benefits
Change in benefit obligation
Benefit obligation, beginning of year	305	290
Service cost	8	10	10
Interest cost	9	12	11
Plan participants' contributions	8	7
Actuarial loss/(gain)	5	10
Effect of plan amendments	(47)	0
Medicare Part D subsidy	1	0
Benefits paid	(28)	(24)
Benefit obligation, end of year	261	305	290
Fair value of plan assets, end of year	260	254	$ 257
Over (Under) funded status	$ (1)	$ (51)