Derivative contracts - Narrative (Detail) - USD ($)	1 Months Ended	6 Months Ended
	Jul. 31, 2016	Jun. 30, 2017	Dec. 31, 2016
Finance and Financial Products [Member]
Derivative [Line Items]
Liabilities		$ 2,494,000,000	$ 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		$ 842,000,000	1,000,000,000
Weighted average remaining life of derivative contracts		3 years 4 months 24 days
Liabilities		$ 2,494,000,000	2,890,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member] | Minimum [Member]
Derivative [Line Items]
Derivative maturity month and year		2018-06
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member] | Maximum [Member]
Derivative [Line Items]
Derivative maturity month and year		2026-01
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] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Assets		$ 123,000,000	142,000,000
Accounts Payable, Accruals and Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Liabilities		$ 139,000,000	$ 145,000,000