Derivative contracts - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finance and Financial Products [Member]
Derivative [Line Items]
Liabilities	$ 2,172,000,000	$ 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	$ 789,000,000	1,000,000,000
Weighted average remaining life of derivative contracts	2 years 10 months 24 days
Liabilities	$ 2,172,000,000	2,890,000,000
Finance and Financial Products [Member] | Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member] | Maximum [Member]
Derivative [Line Items]
Derivative inception month and year	2008-03
Derivative maturity month and year	2026-01
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
Other Assets [Member] | Utilities and Energy [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Assets	$ 142,000,000	142,000,000
Accounts Payable, Accruals and Other Liabilities [Member] | Utilities and Energy [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Liabilities	$ 82,000,000	$ 145,000,000