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Investments - Net Investment Income and Investment Gains (Losses) (Parenthetical) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Net Investment Income [Line Items]        
Net unrealized gains (losses) on trading securities $ 8 $ (4) $ 48 $ (25)
Fixed Maturity Securities [Member]        
Net Investment Income [Line Items]        
Gross realized gains on available-for-sale securities 28 37 64 106
Gross realized losses on available-for-sale securities 31 33 73 84
Fixed Maturity Securities [Member] | Nonredeemable Preferred Stock [Member]        
Net Investment Income [Line Items]        
Net realized gains (losses) on trading securities $ 11 $ 10 $ 53 $ 25