Retroactive reinsurance contracts - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 42,269	$ 41,633	$ 42,441	$ 41,834
Deferred charges under retroactive reinsurance contracts	13,488	13,831	13,747	$ 14,104
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior accident years' events	283	$ 275
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	$ 6,100		$ 6,300