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Investments - OTTI OCI (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Other than Temporary Impairment, Recognized in Accumulated Other Comprehensive Income [Roll Forward]        
Balance, beginning $ 6 $ 8 $ 16 $ 9
Additional Credit Impairments [Abstract]        
On securities previously impaired 0 0 0 0
Reductions [Abstract]        
Increase in cash flows 0 0 0 0
Securities sold, matured, prepaid, or paid down 1 1 11 2
Balance, ending $ 5 $ 7 $ 5 $ 7