Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
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	490.5	$ 686.3
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	149.3	195.6
Asset Pledged as Collateral | Letter of Credit
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	169.8	193.9
Asset Pledged as Collateral | Deposits
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	$ 171.4	$ 296.8