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Investments - OTTI OCI (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]    
Balance, beginning $ 16 $ 9
Additional Credit Impairments [Abstract]    
On securities previously impaired 0 0
Reductions [Abstract]    
Increase in cash flows 0 0
Securities sold, matured, prepaid, or paid down 10 1
Balance, ending $ 6 $ 8