Retroactive reinsurance contracts - Narrative (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 42,937,000,000		$ 42,441,000,000	$ 41,834,000,000	$ 42,937,000,000	$ 24,972,000,000
Deferred charges under retroactive reinsurance contracts	15,278,000,000		13,747,000,000	14,104,000,000	15,278,000,000	$ 8,047,000,000
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses			1,873,000,000	1,436,000,000	11,733,000,000
Incurred losses and loss adjustment expenses, prior accident years' events			1,188,000,000	919,000,000	458,000,000
Liabilities for environmental, asbestos and latent injury claims			12,900,000,000	13,100,000,000
Retroactive Reinsurance [Member] | Deferred Charges Reinsurance Assumed [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses			357,000,000	1,174,000,000	(7,231,000,000)
Incurred losses and loss adjustment expenses, prior accident years' events			810,000,000	1,260,000,000	499,000,000
Retroactive Reinsurance [Member] | Unpaid Losses and Loss Adjustment Expenses [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses			1,516,000,000	262,000,000	18,964,000,000
Incurred losses and loss adjustment expenses, prior accident years' events			378,000,000	(341,000,000)	$ (41,000,000)
AIG [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Percentage of losses and allocated loss adjustment expenses reinsured in excess of retained amount		80.00%
Losses and allocated loss adjustment expenses retained amount		$ 25,000,000,000
Premiums earned		10,200,000,000
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts		16,400,000,000	18,200,000,000	18,200,000,000
Deferred charges under retroactive reinsurance contracts		6,200,000,000	$ 6,300,000,000	$ 6,900,000,000
AIG [Member] | Retroactive Reinsurance [Member] | Deferred Charges Reinsurance Assumed [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses	(1,700,000,000)
AIG [Member] | Retroactive Reinsurance [Member] | Unpaid Losses and Loss Adjustment Expenses [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses	$ 1,800,000,000
AIG [Member] | Maximum [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Losses and allocated loss adjustment expenses reinsured in excess of retained amount		$ 25,000,000,000