Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 8,690
Due after one year through five years - amortized cost	10,009
Due after five years through ten years - amortized cost	607
Due after ten years - amortized cost	2,167
Amortized cost of fixed maturity securities	22,397	$ 22,434
Due in one year or less - fair value	8,762
Due after one year through five years - fair value	10,280
Due after five years through ten years - fair value	660
Due after ten years - fair value	2,674
Fair value of fixed maturity securities	23,411	23,465
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of mortgage-backed securities	924
Amortized cost of fixed maturity securities	924	997
Fair value of mortgage-backed securities	1,035
Fair value of fixed maturity securities	$ 1,035	$ 1,117