Employee Benefits - Overview of the accumulated benefit obligation, fair value of plan assets, funded status and net amount recognized for U.S. and Canadian Other Post-Retirement Benefits (Details) - U.S. and Canadian Other Post-Retirement Benefits - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated projected benefit obligation	$ 99	$ 110
Fair value of plan assets	17	18
Funded status	(82)	(92)
Unrecognized prior-service credit	(1)	(3)
Unrecognized loss	(3)	10
Net amount recognized	$ (86)	$ (85)