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Investments - Net Investment Income and Investment Gains (Losses) (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Net Investment Income [Line Items]        
Net unrealized gains (losses) on trading securities $ 22 $ 8 $ 35 $ 63
Fixed Maturity Securities [Member]        
Net Investment Income [Line Items]        
Gross realized gains on available-for-sale securities 34 68 140 157
Gross realized losses on available-for-sale securities $ 18 $ 24 $ 48 $ 128