Fair value measurements - Other information (Detail) (Significant Unobservable Inputs (Level 3) [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Net Derivative Contract Liabilities [Member] | Equity Index Put Options [Member] | Pricing Model, Option [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, volatility	22.00%
Fair value of liability	$ 7,810
Net Derivative Contract Liabilities [Member] | Credit Default Contracts - States and Municipalities [Member] | Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, credit default spreads	1.20%
Fair value of liability	1,025
Other Fixed Maturities and Equity Securities [Member] | Preferred Stock [Member] | Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, expected duration	10 years
Fair value, unobservable input, discount for transferability restrictions and subordination	0.97%
Fair value of asset	11,307
Other Fixed Maturities and Equity Securities [Member] | Common Stock Warrants [Member] | Pricing Model, Warrant [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, discount for transferability and hedging restrictions	18.00%
Fair value of asset	$ 2,851