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	Jun. 30, 2013	Dec. 31, 2012
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 6,639
Due after one year through five years - amortized cost	11,669
Due after five years through ten years - amortized cost	4,495
Due after ten years - amortized cost	2,418
Total - amortized cost	27,221
Due in one year or less - fair value	6,781
Due after one year through five years - fair value	12,652
Due after five years through ten years - fair value	5,045
Due after ten years - fair value	2,783
Fair Value of Investments	29,505	32,291
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Mortgage-backed securities - amortized cost	2,000
Mortgage-backed securities - fair value	2,244
Fair Value of Investments	$ 2,244	$ 2,587