Pension and Other Postretirement Benefits - Cost of our Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	47	50	140	149
Expected long-term return on plan assets	(66)	(62)	(195)	(186)
Amortization of net loss from past experience	16	17	49	50
Amortization of prior service cost	0	0	0	0
Settlement loss	0	0	6	0
Defined benefit plan cost	(3)	5	1	14
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	0	0	0	0
Interest cost	2	3	7	8
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	(1)	(1)	(2)
Settlement loss	0	0	0	0
Defined benefit plan cost	$ 2	$ 2	$ 6	$ 6