Credit Facilities Schedule of Short term debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Average daily amount of short-term debt outstanding	$ 1	$ 0	$ 0
Weighted daily average interest rate	0.70%	0.60%	0.00%
Maximum amount outstanding during the year	$ 23	$ 11	$ 0