Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plan [Member]
Component of periodic benefit cost
Service cost	$ 1	$ 1
Interest cost	(39)	(35)
Expected return on plan assets	(57)	(56)
Amortization of actuarial loss	(11)	(12)
Net periodic cost	(6)	(8)
Other Postretirement Benefits Plan [Member]
Component of periodic benefit cost
Service cost	0	0
Interest cost	(2)	(2)
Expected return on plan assets	(1)	(2)
Amortization of prior service credit	(1)	(1)
Amortization of actuarial loss	(1)	(1)
Net periodic cost	$ 1	$ 0