Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)		3 Months Ended	9 Months Ended
	Feb. 02, 2017	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses with respect to current accident year events			$ 28,632,000,000	$ 22,737,000,000
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events			(510,000,000)	(1,453,000,000)
Losses and loss adjustment expenses			11,328,000,000	1,041,000,000
Liability for unpaid losses and loss adjustment expenses		$ 100,138,000,000	100,138,000,000	75,469,000,000	$ 76,918,000,000	$ 73,144,000,000
Hurricanes Harvey, Irma and Maria and an Earthquake in Mexico [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses with respect to current accident year events		$ 3,000,000,000
Primary Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events			$ (569,000,000)	(756,000,000)
Reinsurance Operations [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events				$ (697,000,000)
NICO [Member] | AIG [Member] | Retroactive Reinsurance Agreement [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance cash consideration	$ 10,200,000,000
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
Losses and loss adjustment expenses	10,200,000,000
Liability for unpaid losses and loss adjustment expenses	16,400,000,000
Deferred charges reinsurance assumed	6,200,000,000
NICO [Member] | AIG [Member] | Retroactive Reinsurance Agreement [Member] | Maximum [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and allocated loss adjustment expenses reinsured in excess of retained amount	$ 25,000,000,000