Derivative Instruments (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ 8,000,000
Cash flow hedge, reclassified to earnings, net	0	$ 0
Exposure Associated With Collateralization Events	4,000,000		$ 5,000,000
Maximum [Member]
Non-performance Risk Adjustment	$ 1,000,000