Employee Benefits - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Total recognized	$ 37	$ (25)	$ 215
Retirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	3	6	5
Interest cost	78	96	99
Expected return on assets	(122)	(127)	(138)
Actuarial loss (gain)	16	20	11
Prior service (credit) cost	0	0	0
Curtailment	0	0	0
Net periodic benefit cost	(25)	(5)	(23)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial loss (gain)	60	(6)	232
Recognized actuarial (gain) loss	(10)	(13)	(7)
Prior service cost (credit)	0	0	0
Total recognized	50	(19)	225
Postretirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	0	0	1
Interest cost	2	3	4
Expected return on assets			0
Actuarial loss (gain)	(1)	0	(1)
Prior service (credit) cost	0	(1)	0
Curtailment	0	0	(1)
Net periodic benefit cost	1	2	3
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial loss (gain)	(12)	(17)	3
Recognized actuarial (gain) loss	1	0	1
Prior service cost (credit)	(8)	1	(5)
Total recognized	$ (19)	$ (16)	$ (1)