Derivative contracts - Narrative (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Jul. 31, 2016	Sep. 30, 2017	Dec. 31, 2016
Finance and Financial Products [Member]
Derivative [Line Items]
Liabilities		$ 2,187,000,000	$ 2,890,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Collateral posting requirements under contracts with collateral provisions		0
Additional collateral posting requirements		1,100,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Premiums received at contract inception dates		4,200,000,000
Aggregate intrinsic value of equity index put option contracts		$ 640,000,000	1,000,000,000
Weighted average remaining life of derivative contracts		3 years 2 months 12 days
Liabilities		$ 2,187,000,000	2,890,000,000
Maximum [Member] | Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Derivative inception month and year		2008-03
Derivative maturity month and year		2026-01
Minimum [Member] | Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Derivative maturity month and year		2018-06
Credit Default [Member] | Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Payment upon termination of contract	$ 195,000,000
Other Assets [Member] | Utilities and Energy [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Assets		$ 136,000,000	142,000,000
Accounts Payable, Accruals and Other Liabilities [Member] | Utilities and Energy [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Liabilities		$ 131,000,000	$ 145,000,000