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Summary of Amortized Cost, Unrealized Gains and Losses and Fair Value of Available for Sale Securities (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost $ 53,798 $ 47,915
Available-for-sale Securities, Gross unrealized gains 1,071 2,645
Available-for-sale Securities, Gross unrealized losses 1,532 280
Available-for-sale Securities, Fair value 53,337 50,280
Fixed maturity securities [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 53,798 47,842
Available-for-sale Securities, Gross unrealized gains   2,635
Available-for-sale Securities, Gross unrealized losses   276
Available-for-sale Securities, Fair value 53,337 50,201
Fixed maturity securities [Member] | U.S. Treasury securities and obligations of U.S. Government corporations and agencies [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 459 358
Available-for-sale Securities, Gross unrealized gains 28 39
Available-for-sale Securities, Gross unrealized losses 1 1
Available-for-sale Securities, Fair value 486 396
Fixed maturity securities [Member] | Obligations of states, political subdivisions and foreign governments [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 4,026 3,433
Available-for-sale Securities, Gross unrealized gains 290 414
Available-for-sale Securities, Gross unrealized losses 23 2
Available-for-sale Securities, Fair value 4,293 3,845
Fixed maturity securities [Member] | Corporate Securities [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 41,991 37,643
Available-for-sale Securities, Gross unrealized gains 645 2,018
Available-for-sale Securities, Gross unrealized losses 1,403 220
Available-for-sale Securities, Fair value 41,233 39,441
Fixed maturity securities [Member] | Residential mortgage-backed securities [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 2,797 2,788
Available-for-sale Securities, Gross unrealized gains 65 107
Available-for-sale Securities, Gross unrealized losses 39 23
Available-for-sale Securities, Fair value 2,823 2,872
Fixed maturity securities [Member] | Commercial mortgage-backed securities [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 1,731 1,154
Available-for-sale Securities, Gross unrealized gains 9 13
Available-for-sale Securities, Gross unrealized losses 19 6
Available-for-sale Securities, Fair value 1,721 1,161
Fixed maturity securities [Member] | Asset-backed Securities [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost 2,794 2,466
Available-for-sale Securities, Gross unrealized gains 34 44
Available-for-sale Securities, Gross unrealized losses 47 24
Available-for-sale Securities, Fair value $ 2,781 2,486
Equity securities [Member]    
Available-for-Sale Securities    
Available-for-Sale Securities, Amortized cost [1]   73
Available-for-sale Securities, Gross unrealized gains [1]   10
Available-for-sale Securities, Gross unrealized losses [1]   4
Available-for-sale Securities, Fair value [1]   $ 79
[1] As disclosed within Note 3, the Company adopted ASU 2016-01 on January 1, 2018, which required the Company to measure equity securities at fair value with any changes in fair value recognized through net realized investment gains and losses.