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, 2012	Dec. 31, 2011
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 5,960
Due after one year through five years - amortized cost	13,617
Due after five years through ten years - amortized cost	4,472
Due after ten years - amortized cost	2,412
Total - amortized cost	28,834
Due in one year or less - fair value	5,955
Due after one year through five years - fair value	14,656
Due after five years through ten years - fair value	5,171
Due after ten years - fair value	2,948
Fair Value of Investments	31,435	32,188
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Mortgage-backed securities - amortized cost	2,373
Mortgage-backed securities - fair value	2,705
Fair Value of Investments	$ 2,705	$ 2,892