Employee Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic cost for retirement plans and post-retirement plans
Net periodic benefit cost	$ (32)	$ 79	$ (30)
Non-cash pre-tax settlement charge	3	113
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Settlement charge		113
Total recognized	23	(102)	5
Retirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	4	3	3
Interest cost	52	64	71
Expected return on assets	(102)	(108)	(124)
Actuarial loss (gain)	17	12	20
Prior service credit	0	0	0
Net periodic benefit cost	(29)	(29)	(30)
Settlement charge	3	113	4
Total net periodic benefit cost	(26)	84	(26)
Other changes in plan assets and benefit obligations recognized in other comprehensive income, net of tax
Net actuarial loss (gain)	28	(10)	28
Recognized actuarial (gain) loss	(9)	(10)	(15)
Prior service cost	0	0	1
Settlement charge	(2)	(85)	(4)
Total recognized	17	(105)	10
Postretirement Plans
Components of net periodic cost for retirement plans and post-retirement plans
Service cost	0	0	0
Interest cost	1	1	1
Expected return on assets	0	0	0
Actuarial loss (gain)	(2)	(2)	(2)
Prior service credit	(1)	(1)	(1)
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 loss (gain)	1	0	(7)
Recognized actuarial (gain) loss	2	1	1
Prior service cost	1	1	1
Settlement charge	0	0	0
Total recognized	$ 4	$ 2	$ (5)