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	Sep. 30, 2013	Dec. 31, 2012
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 6,733
Due after one year through five years - amortized cost	11,297
Due after five years through ten years - amortized cost	4,411
Due after ten years - amortized cost	2,854
Total - amortized cost	27,213
Due in one year or less - fair value	6,916
Due after one year through five years - fair value	12,249
Due after five years through ten years - fair value	4,970
Due after ten years - fair value	3,252
Fair Value of Investments	29,535	32,291
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Mortgage-backed securities - amortized cost	1,918
Mortgage-backed securities - fair value	2,148
Fair Value of Investments	$ 2,148	$ 2,587