Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities required to be maintained at Lloyd's	$ 261.6
Asset Pledged as Collateral
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	614.2	$ 686.3
Asset Pledged as Collateral | Securities Deposits for Regulatory and Other Purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	168.4	195.6
Asset Pledged as Collateral | Letter of Credit
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	184.2	193.9
Asset Pledged as Collateral | Deposits
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	261.6	$ 296.8
Argo Re
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities and cash on deposit supporting Lloyd's business	$ 136.9