Postretirement Benefit Plans (Rollforward of Obligation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in benefit obligation:
Benefit obligation at beginning of year	$ 840
Benefit obligation at end of year	918	$ 840
Other postretirement benefit plans
Change in benefit obligation:
Benefit obligation at beginning of year	61	58
Service cost	1	1	$ 1
Interest cost	3	2	3
Benefits paid	(4)	(3)
Actuarial loss	3	4
Plan amendment	0	1
Benefit obligation at end of year	$ 64	$ 61	$ 58