Pension and Other Postretirement Benefits - Cost of our Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	$ 1	$ 1	$ 1	$ 1
Interest cost	45	49	93	99
Expected long-term return on plan assets	(64)	(62)	(129)	(124)
Amortization of net loss from past experience	17	16	33	33
Amortization of prior service cost	0	0	0	0
Settlement loss	0	0	6	0
Defined benefit plan cost	(1)	4	4	9
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	0	0	0	0
Interest cost	2	2	5	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	0	0	(1)	(1)
Settlement loss	0	0	0	0
Defined benefit plan cost	$ 2	$ 2	$ 4	$ 4