Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities required to be maintained at Lloyd's		$ 171.4
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Argo Underwriting Agency Limited
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities required to be maintained at Lloyd's		123.7
Asset Pledged as Collateral
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	$ 262.6	490.5
Asset Pledged as Collateral | Securities and cash on deposit for regulatory and other purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	153.4	149.3
Asset Pledged as Collateral | Letter of Credit
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	109.2	169.8
Asset Pledged as Collateral | Deposits
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	$ 0.0	$ 171.4