Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 18,523	$ 19,427
Unrealized gains on fixed maturity securities	665	546
Unrealized losses on fixed maturity securities	(16)	(75)
Fair value of fixed maturity securities	19,172	19,898
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	3,760	4,223
Unrealized gains on fixed maturity securities	47	22
Unrealized losses on fixed maturity securities	(2)	(22)
Fair value of fixed maturity securities	3,805	4,223
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	152	182
Unrealized gains on fixed maturity securities	5	7
Fair value of fixed maturity securities	157	189
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	7,962	7,480
Unrealized gains on fixed maturity securities	70	50
Unrealized losses on fixed maturity securities	(9)	(28)
Fair value of fixed maturity securities	8,023	7,502
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	6,230	7,055
Unrealized gains on fixed maturity securities	477	408
Unrealized losses on fixed maturity securities	(3)	(23)
Fair value of fixed maturity securities	6,704	7,440
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	419	487
Unrealized gains on fixed maturity securities	66	59
Unrealized losses on fixed maturity securities	(2)	(2)
Fair value of fixed maturity securities	$ 483	$ 544