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Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]        
Other-than-temporary impairment losses for debt securities $ 8 $ 1 $ 25 $ 2