Segment Information - Identifiable Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 10,034.4	$ 10,317.8
Assets held-for-sale	2,066.2	0.0
Argo Underwriting Agency Limited | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Assets held-for-sale	2,066.2
Operating Segments | U.S. Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	5,815.0	5,800.1
Operating Segments | International Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	3,791.6	3,932.3
Operating Segments | International Operations | Argo Underwriting Agency Limited | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Segment Reporting, Asset Reconciling Item [Line Items]
Assets held-for-sale	2,066.2
Operating Segments | Run-off Lines
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	284.4	314.7
Corporate and Other
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable assets	$ 143.4	$ 270.7