Retroactive reinsurance contracts - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 38,256	$ 40,966	$ 42,441	$ 41,834
Deferred charges - retroactive reinsurance	10,639	12,441	13,747	$ 14,104
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts	845	907	$ 1,188
Liabilities for environmental, asbestos and latent injury claims	12,300	12,500
AIG [Member] | Retroactive Reinsurance [Member] | NICO [Member]
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	15,800	17,700
Retroactive reinsurance contracts, claims paid	1,200
Deferred charges - retroactive reinsurance	$ 4,500	$ 5,400