Benefit Plans - Components of Net Periodic Postretirement Health Care Costs (Details) - Postretirement Health Care Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 6	$ 7
Interest cost	15	14	15
Net loss from experience differences	6	15	14
Prior service credit	(38)	(39)	(40)
Net periodic postretirement health care costs/(benefit)	$ (12)	$ (4)	$ (4)