Derivative Instruments (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ 21,000,000
Cash flow hedge, reclassified to earnings, net	0	$ 0
Exposure Associated With Collateralization Events	0		$ 0
Maximum [Member]
Non-performance Risk Adjustment	$ 1,000,000