Employee Pension and Other Postretirement Benefits - Changes in Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Accumulated benefit obligation	$ 2,679	$ 2,717
Change in benefit obligation
Benefit obligation, beginning of year	2,817	2,263
Service cost	96	75
Interest cost	110	105	$ 99
Plan participants' contributions	0	0
Actuarial loss/(gain)	(114)	607
Effect of plan amendments	0	(150)
Medicare Part D subsidy	0	0
Benefits paid	(93)	(83)
Benefit obligation, end of year	2,816	2,817	2,263
Fair value of plan assets, end of year	2,992	2,897	2,716
Over (Under) funded status	176	80
Other Benefits
Change in benefit obligation
Benefit obligation, beginning of year	290	270
Service cost	10	10
Interest cost	12	11	11
Plan participants' contributions	7	7
Actuarial loss/(gain)	10	42
Effect of plan amendments	0	(29)
Medicare Part D subsidy	0	1
Benefits paid	(24)	(22)
Benefit obligation, end of year	305	290	270
Fair value of plan assets, end of year	254	257	$ 245
Over (Under) funded status	$ (51)	$ (33)