Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities required to be maintained at Lloyd's	$ 173.9
Asset Pledged as Collateral
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	437.6	$ 490.5
Asset Pledged as Collateral | Securities Deposits for Regulatory and Other Purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	142.8	149.3
Asset Pledged as Collateral | Letter of Credit
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	120.9	169.8
Asset Pledged as Collateral | Deposits
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	$ 173.9	$ 171.4