Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)		3 Months Ended
	Feb. 02, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses for (reductions of) prior accident years excluding discount and deferred charge changes		$ 118,000,000	$ (372,000,000)
Losses and loss adjustment expenses		10,283,000,000	712,000,000
Unpaid losses and loss adjustment expenses		$ 93,898,000,000	$ 74,562,000,000	$ 76,918,000,000	$ 73,144,000,000
NICO [Member] | AIG [Member] | Retroactive Reinsurance Agreement [Member]
Liability for Claims and Claims Adjustment Expense [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
Reinsurance cash consideration	10,200,000,000
Premiums earned	10,200,000,000
Losses and loss adjustment expenses	10,200,000,000
Unpaid losses and loss adjustment expenses	16,400,000,000
Deferred charges reinsurance assumed	6,200,000,000
Maximum [Member] | NICO [Member] | AIG [Member] | Retroactive Reinsurance Agreement [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