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Fair Value of Assets and Liabilities (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Fair Value Disclosures [Abstract]        
Transfers Between Level 1 and Level 2 $ 0 $ 0 $ 0 $ 0