Employee Benefits - Net Postretirement Benefit Cost and the Amount Recognized in OCI for All Funded and Unfunded Plans (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost of benefits earned	$ (1)	$ (1)	$ (1)
Interest cost on APBO	(3)	(2)	(3)
Expected return on plan assets	(2)	(2)	(3)
Amortization of prior service credit	(1)	(1)	(1)
Amortization of gains	0	0	0
Net pension cost (benefit)	1	0	0
Other changes in plan assets and benefit obligations recognized in OCI:
Net (gain) loss	(4)	(4)	(6)
Amortization of prior service credit	1	1	1
Amortization of losses	0	0	0
Total recognized in comprehensive income	(3)	(3)	(5)
Total recognized in net postretirement benefit cost and comprehensive income	$ (2)	$ (3)	$ (5)