Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plan [Member]
Component of periodic benefit cost
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	(36)	(49)	(71)	(98)
Expected return on plan assets	(59)	(80)	(115)	(159)
Amortization of actuarial loss	(13)	(15)	(25)	(30)
Settlements	0	750	0	750
Net periodic cost (benefit)	(9)	735	(17)	721
Other Postretirement Benefits Plan [Member]
Component of periodic benefit cost
Service cost	0	0	0
Interest cost	(1)	(2)	(3)	(4)
Expected return on plan assets	(1)	(2)	(3)	(4)
Amortization of prior service credit	(2)	(2)	(3)	(3)
Amortization of actuarial loss	(2)	(1)	(3)	(2)
Settlements		0
Net periodic cost (benefit)	$ 0	$ (1)	$ 0	$ (1)