Run-off Lines - Gross Reserves for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ 65.5	$ 63.8
Gross reserves - Run-off Lines	233.1	261.2
Reinsurance assumed
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	31.1	29.6
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	34.4	34.2
Risk-management
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	144.6	162.6
Run-off reinsurance lines
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	0.4	0.5
Other run-off lines
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	$ 22.6	$ 34.3