Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Summary of Investment Holdings [Line Items]
Total - amortized cost	$ 27,547	$ 27,764
Total - fair value	29,431	29,370
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	7,969
Due after one year through five years - amortized cost	12,639
Due after five years through ten years - amortized cost	2,628
Due after ten years - amortized cost	2,655
Due in one year or less - fair value	8,075
Due after one year through five years - fair value	13,450
Due after five years through ten years - fair value	2,875
Due after ten years - fair value	3,155
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Mortgage-backed securities - amortized cost	1,656
Mortgage-backed securities - fair value	$ 1,876