Fair value measurements - Other information (Detail) (Significant Unobservable Inputs (Level 3) [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
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, unobservable input, credit spreads	0.98%
Fair value of liability	$ 478
Pricing Model, Option [Member] | Net Derivative Contract Liabilities [Member] | Equity Index Put Options [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, volatility	21.00%
Fair value of liability	5,355
Other Investments [Member] | Preferred Stock [Member] | Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value, unobservable input, expected duration	6 years
Fair value, unobservable input, discount for transferability restrictions and subordination	0.97%
Fair value of asset	11,857
Other Investments [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	21.00%
Fair value of asset	$ 4,228