Run-off Lines - Gross Reserves for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ 63.8	$ 59.3
Gross reserves - Run-off Lines	261.2	236.5
Reinsurance assumed
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	29.6	29.4
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	34.2	29.9
Risk-management
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	162.6	162.4
Run-off reinsurance lines
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	0.5	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	$ 34.3	$ 14.3