Pension and Other Postretirement Benefits - Expenses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 25	$ 1	$ 119	$ 4
Multiemployer contribution	0	16	0	46
Cost allocation	0	(5)	(1)	(15)
Pension and other postretirement benefits cost	22	22	60	65
Pension benefits \| Cumulative Direct Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	22	1	61	4
Other postretirement benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	0	17	0
Multiemployer contribution	0	4	0	15
Cost allocation	0	(2)	0	(4)
Pension and other postretirement benefits cost	5	6	17	19
Other postretirement benefits \| Cumulative Direct Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 5	$ 0	$ 17	$ 0