Employee Benefits - Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Total recognized	$ (41)	$ 37	$ (25)
Retirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	3	3	6
Interest cost	74	78	96
Expected return on assets	(126)	(122)	(127)
Actuarial loss (gain)	18	16	20
Prior service (credit) cost	0	0	0
Other	8	0	0
Net periodic benefit cost	(23)	(25)	(5)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial (gain) loss	(20)	60	(6)
Recognized actuarial (gain) loss	(12)	(10)	(13)
Prior service (credit) cost	0	0	0
Other	(7)
Total recognized	(39)	50	(19)
Postretirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	0	0	0
Interest cost	2	2	3
Expected return on assets	0	0	0
Actuarial loss (gain)	(2)	(1)	0
Prior service (credit) cost	(2)	0	(1)
Other	0	0	0
Net periodic benefit cost	(2)	1	2
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial (gain) loss	(3)	(12)	(17)
Recognized actuarial (gain) loss	1	1	0
Prior service (credit) cost	1	(8)	1
Other
Total recognized	$ (1)	$ (19)	$ (16)