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Investments - Net Investment Income and Investment Gains (Losses) (Parenthetical) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Net Investment Income [Line Items]      
Net unrealized gains (losses) on trading securities $ (121) $ 39 $ 44
Fixed Maturity Securities [Member]      
Net Investment Income [Line Items]      
Gross realized gains on available-for-sale securities 168 187 209
Gross realized losses on available-for-sale securities 164 $ 65 $ 160
Fixed Maturity Securities [Member] | Nonredeemable Preferred Stock [Member]      
Net Investment Income [Line Items]      
Net realized gains (losses) on trading securities $ (73)