Recently Issued Accounting Standards (Narrative) (Detail)	9 Months Ended
Sep. 30, 2012
Significant Accounting Policies [Line Items]
Intangible assets more likely-than-not threshold	50.00%

Indefinite-Lived Intangible Asset More Likely Than Not Threshold

No definition available.

Name:	agii_IndefiniteLivedIntangibleAssetMoreLikelyThanNotThreshold
Namespace Prefix:	agii_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration