Retroactive reinsurance contracts - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 40,966,000,000	$ 42,441,000,000	$ 41,834,000,000	$ 42,937,000,000
Deferred charges under retroactive reinsurance contracts	12,441,000,000	13,747,000,000	14,104,000,000	$ 15,278,000,000
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	907,000,000	1,188,000,000	$ 919,000,000
Liabilities for environmental, asbestos and latent injury claims	12,500,000,000	12,900,000,000
AIG [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 17,700,000,000	18,200,000,000
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
Deferred charges under retroactive reinsurance contracts	5,400,000,000	$ 6,300,000,000
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