Pension and Other Postretirement Benefits - Cost of our Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	42	47	126	140
Expected long-term return on plan assets	(64)	(66)	(192)	(195)
Amortization of net loss from past experience	19	16	54	49
Amortization of prior service cost	0	0	0	0
Settlement loss	0	0	0	6
Net plan cost	(3)	(3)	(11)	1
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	0	0	0	0
Interest cost	2	2	6	7
Expected long-term return on plan assets	0	0	0	0
Amortization of net loss from past experience	1	0	1	0
Amortization of prior service cost	(1)	0	(2)	(1)
Settlement loss	0	0	0	0
Net plan cost	$ 2	$ 2	$ 5	$ 6