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Fair Value (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
item
Dec. 31, 2016
USD ($)
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 $ 89 $ 181  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | item 121 246  
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 $ 0    
Other Investment Income [Member]      
Fair Value Disclosures [Line Items]      
Investment Income, Interest and Dividend $ 7 $ 8 $ 7
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