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Fair Value (Narrative) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
item
Fair Value Disclosures [Line Items]    
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount $ 0 $ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount 0 0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0 0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value $ 181 $ 232
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | item 246 226
Available-for-sale Securities [Member]    
Fair Value Disclosures [Line Items]    
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held $ 1  
Interest Rate Swap [Member]    
Fair Value Disclosures [Line Items]    
Credit risk valuation adjustment, decrease in fair value of liability 1 $ 3
Credit risk valuation adjustment, decrease in fair value of liability, net of tax $ 1 $ 2