Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)		6 Months Ended
	Feb. 02, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events		$ (199,000,000)	$ (1,071,000,000)
Losses and loss adjustment expenses		10,532,000,000	883,000,000
Liability for unpaid losses and loss adjustment expenses		95,307,000,000	74,708,000,000	$ 76,918,000,000	$ 73,144,000,000
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		(532,000,000)	(452,000,000)
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			$ (619,000,000)
Reinsurance Operations [Member] | Personal Injury Coverage [Member] | United Kingdom [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events		$ 215,000,000
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