Pension and Other Postretirement Benefits - Cost of our Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	$ 1	$ 0
Interest cost	42	48
Expected long-term return on plan assets	(64)	(65)
Amortization of net loss from past experience	17	16
Amortization of prior service cost	0	0
Settlement loss	0	6
Net plan cost	(4)	5
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Service cost	0	0
Interest cost	2	3
Expected long-term return on plan assets	0	0
Amortization of net loss from past experience	0	0
Amortization of prior service cost	0	(1)
Settlement loss	0	0
Net plan cost	$ 2	$ 2