Derivative contracts - Narrative (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Finance and Financial Products [Member]
Derivative [Line Items]
Liabilities	$ 4,667,000,000		$ 5,331,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Collateral posting requirements under contracts with collateral provisions	0		0
Additional collateral posting requirements	1,100,000,000
Liabilities	4,667,000,000		5,331,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Aggregate intrinsic value of equity index put option contracts	1,300,000,000		1,700,000,000
Maturity date, range, start	June 2018
Maturity date, range, end	January 2026
Weighted average remaining life of derivative contract	6 years 2 months 12 days
Notional value	30,567,000,000	[1]	32,095,000,000	[1]
Liabilities	4,512,000,000		4,667,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [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	17 years
Notional value	7,792,000,000
Railroad, Utilities and Energy [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivative [Line Items]
Assets	86,000,000		87,000,000
Railroad, Utilities and Energy [Member] | Not Designated as Hedging Instrument [Member] | Accounts Payable, Accruals and Other Liabilities [Member]
Derivative [Line Items]
Liabilities	$ 146,000,000		$ 208,000,000

[1] Represents the aggregate undiscounted amount payable at the contract expiration dates assuming that the value of the related index is zero at each contract's expiration date.