Retroactive reinsurance contracts - Narrative (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 41,786	$ 41,364	$ 42,441	$ 41,834
Deferred charges under retroactive reinsurance contracts	13,012	13,359	13,747	$ 14,104
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	866	$ 825
NICO [Member] | AIG [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Maximum contractual liability under contract	20,000
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	18,200		18,200
Deferred charges under retroactive reinsurance contracts	$ 5,800		$ 6,300