Derivative contracts - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative contract gains (losses)	$ 1,484,000,000	$ (300,000,000)	$ 718,000,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Collateral posting requirements under contracts with collateral provisions	0
Not Designated as Hedging Instrument [Member] | Utilities and Energy [Member]
Derivative [Line Items]
Assets	145,000,000	172,000,000
Liabilities	76,000,000	111,000,000
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Derivative [Line Items]
Derivative contract gains (losses)	1,484,000,000	(300,000,000)	$ 718,000,000
Notional value of contracts that expired in current fiscal year	$ 12,300,000,000
Derivative maturity month and year	2023-02
Weighted average remaining life of derivative contracts	1 year 9 months 18 days
Premiums received at contract inception dates	$ 2,500,000,000
Aggregate intrinsic value of equity index put option contracts	397,000,000	1,653,000,000
Liabilities	$ 968,000,000	$ 2,452,000,000
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member] | Maximum [Member]
Derivative [Line Items]
Derivative inception month and year	2008-03