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Derivative Instruments (Narrative) (Details) - USD ($)
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months $ 6,000,000  
Cash flow hedge, reclassified to earnings, net 0 $ 0
Exposure Associated With Collateralization Events 4,000,000  
Maximum [Member]    
Collateral Requirement If Netting Agreements Not In Place 1,000,000  
Non-performance Risk Adjustment $ 1,000,000