Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plan [Member]
Component of periodic benefit cost
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	(32)	(40)	(64)	(79)
Expected return on plan assets	(54)	(56)	(108)	(113)
Amortization of prior service credit	0	0	0	0
Amortization of actuarial loss	(15)	(11)	(30)	(22)
Net periodic cost (benefit)	(6)	(4)	(12)	(10)
Other Postretirement Benefits Plan [Member]
Component of periodic benefit cost
Service cost	0	0	0	0
Interest cost	(1)	(2)	(3)	(4)
Expected return on plan assets	(1)	(1)	(2)	(2)
Amortization of prior service credit	(1)	(2)	(3)	(3)
Amortization of actuarial loss	(1)	(2)	(3)	(3)
Net periodic cost (benefit)	$ 0	$ 1	$ 1	$ 2