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Fair Value (Narrative) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Fair Value Disclosures [Line Items]      
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount $ 0   $ 0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount $ 0   $ 0
Interest Rate Swap [Member]      
Fair Value Disclosures [Line Items]      
Credit risk valuation adjustment, decrease in fair value of liability   $ 1  
Credit risk valuation adjustment, decrease in fair value of liability, net of tax   $ 1