Derivative contracts - Narrative (Detail) (Not Designated as Hedging Instrument [Member], USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Finance and Financial Products [Member]
Derivative [Line Items]
Collateral posting requirements under contracts with collateral provisions	$ 0	$ 0
Additional collateral posting requirements	1,100,000,000
Liabilities	5,095,000,000	5,331,000,000
Finance and Financial Products [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Aggregate intrinsic value of equity index put option contracts	1,600,000,000	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 9 months
Liabilities	4,800,000,000	4,667,000,000
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	17 years 6 months
Notional value	7,800,000,000
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Derivative [Line Items]
Assets	93,000,000	87,000,000
Railroad, Utilities and Energy [Member] | Accounts Payable, Accruals and Other Liabilities [Member]
Derivative [Line Items]
Liabilities	$ 160,000,000	$ 208,000,000