Employee Benefits - Changes in APBO (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
PBO/APBO at beginning of year	$ 69	$ 67
Service cost	1	1	$ 1
Interest cost	3	2	3
Plan participants’ contributions	1	2
Actuarial losses (gains)	3	(1)
Benefit payments	(8)	(6)
Plan acquisitions	0	4
PBO/APBO at end of year	$ 69	$ 69	$ 67