Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (Fixed Maturities [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 7,986
Due after one year through five years - amortized cost	12,704
Due after five years through ten years - amortized cost	2,587
Due after ten years - amortized cost	2,797
Total - amortized cost	27,843	27,764
Due in one year or less - fair value	8,069
Due after one year through five years - fair value	13,431
Due after five years through ten years - fair value	2,864
Due after ten years - fair value	3,221
Fair Value of Investments	29,572	29,370
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Mortgage-backed securities - amortized cost	1,769
Mortgage-backed securities - fair value	$ 1,987