Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Amortized cost and estimated fair value of securities with fixed maturities [Line Items]
Due in one year or less - amortized cost	$ 8,466
Due after one year through five years - amortized cost	13,848
Due after five years through ten years - amortized cost	4,503
Due after ten years - amortized cost	2,480
Mortgage-backed securities - amortized cost	2,464
Total - amortized cost	31,761	31,839
Due in one year or less - fair value	8,591
Due after one year through five years - fair value	14,672
Due after five years through ten years - fair value	4,946
Due after ten years - fair value	3,034
Mortgage-backed securities - fair value	2,805
Fair Value	$ 34,048	$ 34,883