Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1	$ 1
Interest cost	(24)	(32)
Expected return on plan assets	(51)	(54)
Amortization of prior service credit	0	0
Amortization of actuarial loss	(17)	(15)
Net periodic cost (benefit)	(9)	(6)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	(1)	(2)
Expected return on plan assets	(1)	(1)
Amortization of prior service credit	(2)	(2)
Amortization of actuarial loss	(2)	(2)
Net periodic cost (benefit)	$ 0	$ 1