Derivative contracts - Narrative (Detail) (USD $)	1 Months Ended							9 Months Ended		9 Months Ended
	Aug. 31, 2012 Finance and Financial Products [Member] Credit Default Contracts - States and Municipalities [Member]	Sep. 30, 2012 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member]	Dec. 31, 2011 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member]	Sep. 30, 2012 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member] Other Assets [Member]		Dec. 31, 2011 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member] Other Assets [Member]		Sep. 30, 2012 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member] Equity Index Put Options [Member]	Dec. 31, 2011 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member] Equity Index Put Options [Member]	Sep. 30, 2012 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member] Credit Default Contracts - States and Municipalities [Member]	Sep. 30, 2012 Finance and Financial Products [Member] Not Designated as Hedging Instrument [Member] Credit Default Contracts - Individual Corporate and High Yield Corporate [Member]	Sep. 30, 2012 Railroad, Utilities and Energy [Member] Not Designated as Hedging Instrument [Member] Other Assets [Member]	Dec. 31, 2011 Railroad, Utilities and Energy [Member] Not Designated as Hedging Instrument [Member] Other Assets [Member]	Sep. 30, 2012 Railroad, Utilities and Energy [Member] Not Designated as Hedging Instrument [Member] Accounts Payable, Accruals and Other Liabilities [Member]	Dec. 31, 2011 Railroad, Utilities and Energy [Member] Not Designated as Hedging Instrument [Member] Accounts Payable, Accruals and Other Liabilities [Member]
Derivative [Line Items]
Aggregate intrinsic value of equity index put option contracts								$ 5,400,000,000	$ 6,200,000,000
Maturity date, range, start								June 2018		2019	December 2012
Maturity date, range, end								January 2026		2054	December 2013
Weighted average remaining life of derivative contract								8 years 2 months 12 days		19 years
Terminated derivative contracts, notional values	8,250,000,000
Notional amount of derivatives expiring within one year											6,100,000,000
Collateral posting requirements under contracts with collateral provisions		150,000,000	238,000,000
Additional collateral posting requirements		1,100,000,000
Assets				310,000,000	[1]	256,000,000	[1]					33,000,000	71,000,000
Liabilities		$ 10,090,000,000	$ 10,139,000,000					$ 9,517,000,000	$ 8,499,000,000					$ 230,000,000	$ 336,000,000

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