Derivative contracts - Narrative (Detail) (Not Designated as Hedging Instrument [Member], USD $)	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Finance and Financial Products [Member]
Derivative [Line Items]
Collateral posting requirements under contracts with collateral provisions	$ 0		$ 40,000,000
Additional collateral posting requirements	1,100,000,000
Liabilities	5,847,000,000		7,933,000,000
Finance and Financial Products [Member] | Other Assets [Member]
Derivative [Line Items]
Assets	12,000,000	[1]	171,000,000	[1]
Finance and Financial Products [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Aggregate intrinsic value of equity index put option contracts	2,300,000,000		3,900,000,000
Maturity date, range, start	June 2018
Maturity date, range, end	January 2026
Weighted average remaining life of derivative contract	7 years 3 months
Liabilities	5,355,000,000		7,502,000,000
Finance and Financial Products [Member] | Credit Default Contracts - Individual Corporate and High Yield Corporate [Member]
Derivative [Line Items]
Notional amount of derivatives expiring within one year	718,000,000
Finance and Financial Products [Member] | Credit Default Contracts - States and Municipalities [Member]
Derivative [Line Items]
Maturity date, range, start	2019
Maturity date, range, end	2054
Weighted average remaining life of derivative contract	18 years
Notional value	7,800,000,000
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Derivative [Line Items]
Assets	61,000,000		49,000,000
Railroad, Utilities and Energy [Member] | Accounts Payable, Accruals and Other Liabilities [Member]
Derivative [Line Items]
Liabilities	$ 176,000,000		$ 234,000,000

[1] Included in other assets of finance and financial products businesses.