Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 8,117
Due after one year through five years - amortized cost	9,898
Due after five years through ten years - amortized cost	756
Due after ten years - amortized cost	2,183
Amortized cost of fixed maturity securities	21,891	$ 22,434
Due in one year or less - fair value	8,191
Due after one year through five years - fair value	10,204
Due after five years through ten years - fair value	815
Due after ten years - fair value	2,672
Fair value of fixed maturity securities	22,933	23,465
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of mortgage-backed securities	937
Amortized cost of fixed maturity securities	937	997
Fair value of mortgage-backed securities	1,051
Fair value of fixed maturity securities	$ 1,051	$ 1,117