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	Dec. 31, 2011	Dec. 31, 2010
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 6,969
Due after one year through five years - amortized cost	13,890
Due after five years through ten years - amortized cost	4,192
Due after ten years - amortized cost	2,433
Total - amortized cost	30,048
Due in one year or less - fair value	7,063
Due after one year through five years - fair value	14,487
Due after five years through ten years - fair value	4,823
Due after ten years - fair value	2,923
Fair Value of Investments	32,188	34,883
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Fair Value of Investments	2,892	3,139
Mortgage-backed securities - amortized cost	2,564
Mortgage-backed securities - fair value	$ 2,892