Run-off Lines - Gross Reserves for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ 59.3	$ 52.6
Gross reserves - Run-off Lines	236.5	253.5
Reinsurance assumed
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	29.4	26.7
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	29.9	25.9
Risk-management
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross reserves - Run-off Lines	162.4	188.1
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	$ 14.3	$ 12.3