Pension and Other Postretirement Benefits - Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 73	$ 2	$ 94	$ 3
Multiemployer contribution	0	14	0	30
Cost allocation	0	(5)	(1)	(10)
Pension and other postretirement benefits cost	18	21	38	43
Pension benefits | Cumulative Direct Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	18	2	39	3
Other postretirement benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	6	0	12	0
Multiemployer contribution	0	6	0	11
Cost allocation	0	(1)	0	(2)
Pension and other postretirement benefits cost	6	7	12	13
Other postretirement benefits | Cumulative Direct Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 6	$ 0	$ 12	$ 0