Employee Benefit Plans - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets contributions by employer	$ 4
Total estimated contributions by employer during fiscal year	25
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year	21
Postretirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets contributions by employer	0
Total estimated contributions by employer during fiscal year	2
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year	2
Postemployment Retirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plan assets contributions by employer	10
Total estimated contributions by employer during fiscal year	39
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year	29
Net benefit cost	$ 6	$ 5