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Significant Accounting Policies (Narrative) (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Accounting Policies [Abstract]    
Federal corporate tax rate, percent 21.00% 35.00%
Accounting Standards Update 2018-02 [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
New accounting pronouncement, effect of adoption, quantification $ 128 $ 128