Employee Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Settlement charge	$ 113
Total recognized	(102)	$ 5	$ (41)
Retirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	3	3	3
Interest cost	64	71	74
Expected return on assets	(108)	(124)	(126)
Actuarial loss (gain)	12	20	18
Prior service credit	0	0	0
Total net periodic benefit cost	(29)	(30)	(31)
Settlement charge	113	4	8
Total net periodic benefit cost	84	(26)	(23)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial (gain) loss	(10)	28	(20)
Recognized actuarial (gain) loss	(10)	(15)	(12)
Prior service (credit) cost	0	1	0
Settlement charge	(85)	(4)	(7)
Total recognized	(105)	10	(39)
Postretirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	0	0	0
Interest cost	1	1	2
Expected return on assets	0	0	0
Actuarial loss (gain)	(2)	(2)	(2)
Prior service credit	(1)	(1)	(2)
Total net periodic benefit cost	(2)	(2)	(2)
Settlement charge	0	0	0
Total net periodic benefit cost	(2)	(2)	(2)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial (gain) loss	0	(7)	(3)
Recognized actuarial (gain) loss	1	1	1
Prior service (credit) cost	1	1	1
Settlement charge	0	0	0
Total recognized	$ 2	$ (5)	$ (1)