Employee Benefit Plans - Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 101
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	103	$ 281
Employer contributions	103	280
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	0	0
Employer contributions	$ 0	$ 0