Pension and Other Postretirement Benefits - Cost of our Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	$ 0	$ 1	$ 1	$ 1
Interest cost	42	45	84	93
Expected long-term return on plan assets	(64)	(64)	(128)	(129)
Amortization of net loss from past experience	18	17	35	33
Amortization of prior service cost	0	0	0	0
Settlement loss	0	0	0	6
Net plan cost	(4)	(1)	(8)	4
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	0	0	0	0
Interest cost	2	2	4	5
Expected long-term return on plan assets	0	0	0	0
Amortization of net loss from past experience	0	0	0	0
Amortization of prior service cost	(1)	0	(1)	(1)
Settlement loss	0	0	0	0
Net plan cost	$ 1	$ 2	$ 3	$ 4