Retroactive reinsurance contracts - Narrative (Detail) - USD ($)		3 Months Ended	12 Months Ended
	Feb. 02, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts		$ 42,937,000,000	$ 41,834,000,000	$ 42,937,000,000	$ 24,972,000,000	$ 24,058,000,000
Deferred charges under retroactive reinsurance contracts		15,278,000,000	14,104,000,000	15,278,000,000	8,047,000,000	$ 7,687,000,000
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses			1,436,000,000	11,733,000,000	1,713,000,000
Incurred losses and loss adjustment expenses, prior years' contracts			919,000,000	458,000,000	451,000,000
Liabilities for environmental, asbestos and latent injury claims		14,000,000,000	13,100,000,000	14,000,000,000
Unpaid Losses and Loss Adjustment Expenses [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses			262,000,000	18,964,000,000	2,073,000,000
Incurred losses and loss adjustment expenses, prior years' contracts			(341,000,000)	(41,000,000)	(63,000,000)
Deferred Charges Reinsurance Assumed [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses			1,174,000,000	(7,231,000,000)	(360,000,000)
Incurred losses and loss adjustment expenses, prior years' contracts			1,260,000,000	499,000,000	$ 514,000,000
NICO [Member] | 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	18,200,000,000
Deferred charges under retroactive reinsurance contracts	6,200,000,000	7,500,000,000	$ 6,900,000,000	$ 7,500,000,000
NICO [Member] | AIG [Member] | Retroactive Reinsurance [Member] | Maximum [Member]
Effects of Reinsurance [Line Items]
Losses and allocated loss adjustment expenses reinsured in excess of retained amount	$ 25,000,000,000
NICO [Member] | Unpaid Losses and Loss Adjustment Expenses [Member] | AIG [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses		1,800,000,000
NICO [Member] | Deferred Charges Reinsurance Assumed [Member] | AIG [Member] | Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses		$ (1,700,000,000)