Pension and Other Postretirement Benefit Plans - Narrative (Details) - Other Postretirement Benefits - USD ($) $ in Millions	3 Months Ended			6 Months Ended		75 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2011	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Reduction in postretirement benefit obligation			$ 138
Net periodic cost	$ (3)	$ (2)		$ (5)	$ (4)	$ 103