Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Unrealized losses less than one year	$ 0.1	$ 0.1
Unrealized losses one year or greater	$ 0.1	$ 0.1

Investment securities continuous unrealized loss position less than twelve months fair value.

No definition available.

Name:	agii_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	agii_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant