Benefit Plans - Postretirement Benefit Plans (Details) - Postretirement Benefit Plans - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2	$ 2	$ 3	$ 3
Interest cost	3	3	6	7
Net loss from experience differences	1	1	4	3
Prior service credit	(7)	(9)	(15)	(19)
Net periodic benefit costs	$ (1)	$ (3)	$ (2)	$ (6)