Retroactive reinsurance contracts - Narrative (Detail) - USD ($)		3 Months Ended
	Feb. 02, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts		$ 42,344,000,000	$ 42,937,000,000	$ 40,587,000,000	$ 24,972,000,000
Deferred charges under retroactive reinsurance contracts		15,007,000,000	15,278,000,000	13,830,000,000	$ 8,047,000,000
Retroactive Reinsurance [Member]
Effects of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses, prior years' contracts		241,000,000		$ 11,000,000
NICO [Member] | AIG [Member] | Retroactive Reinsurance Agreement [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
Deferred charges under retroactive reinsurance contracts	6,200,000,000	7,400,000,000	7,500,000,000
Net contract liability		$ 18,200,000,000	18,200,000,000
Increase in ultimate liabilities under reinsurance contract			$ 1,800,000,000
NICO [Member] | AIG [Member] | Retroactive Reinsurance Agreement [Member] | Maximum [Member]
Effects of Reinsurance [Line Items]
Losses and allocated loss adjustment expenses reinsured in excess of retained amount	$ 25,000,000,000