Retroactive reinsurance contracts - Narrative (Detail) - USD ($)		3 Months Ended	12 Months Ended
	Feb. 02, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts		$ 42,937,000,000	$ 42,937,000,000	$ 24,972,000,000	$ 24,058,000,000	$ 24,702,000,000
Deferred charges under retroactive reinsurance contracts		15,278,000,000	15,278,000,000	8,047,000,000	7,687,000,000	$ 7,772,000,000
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts			458,000,000	451,000,000	$ 631,000,000
Liabilities for environmental, asbestos and latent injury claims		14,000,000,000	14,000,000,000	$ 13,700,000,000
NICO [Member] | Retroactive Reinsurance Agreement [Member] | AIG [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
Deferred charges under retroactive reinsurance contracts	6,200,000,000	7,500,000,000	7,500,000,000
Increase in ultimate liabilities under reinsurance contract		1,800,000,000
Increase in deferred charge asset		1,700,000,000
Net contract liability		$ 10,700,000,000	$ 10,700,000,000
NICO [Member] | Retroactive Reinsurance Agreement [Member] | AIG [Member] | Maximum [Member]
Effects of Reinsurance [Line Items]
Losses and allocated loss adjustment expenses reinsured in excess of retained amount	$ 25,000,000,000