Benefit Plans - Postretirement Benefit Plans (Details) - Postretirement Benefit Plans - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1
Interest cost	3	4
Net loss from experience differences	3	2
Prior service credit	(8)	(10)
Net periodic benefit costs	$ (1)	$ (3)