Fair value measurements - Other information (Detail) (Significant Unobservable Inputs (Level 3) [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Discounted Cash Flow Method [Member] | Net Derivative Contract Liabilities [Member] | Credit Default Contracts - States and Municipalities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value of liability	$ 421
Fair value, unobservable input, credit spreads	0.85%
Pricing Model, Option [Member] | Net Derivative Contract Liabilities [Member] | Equity Index Put Options [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value of liability	7,502
Fair value, unobservable input, volatility	21.00%
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 of asset	11,860
Fair value, unobservable input, expected duration	10 years
Fair value, unobservable input, discount for transferability restrictions and subordination	0.97%
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 of asset	$ 3,890
Fair value, unobservable input, discount for transferability and hedging restrictions	19.00%