Pension and Other Postretirement Benefits - Cost of our Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Interest cost	$ 48	$ 50
Expected long-term return on plan assets	(65)	(62)
Amortization of net loss from past experience	16	17
Amortization of prior service cost	0	0
Settlement loss	6	0
Defined benefit plan cost	5	5
Other Postretirement Benefits [Member]
Components of net pension and other postretirement benefits cost [Abstract]
Interest cost	3	3
Expected long-term return on plan assets	0	0
Amortization of net loss from past experience	0	0
Amortization of prior service cost	(1)	(1)
Settlement loss	0	0
Defined benefit plan cost	$ 2	$ 2