Run-off Lines - Underwriting Losses for Run-Off Lines (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	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	$ (14.7)	$ (17.4)	$ (8.3)
Total underwriting loss - Run-off Lines	(44.7)	(12.8)	(14.2)
Reinsurance assumed
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(4.7)	(5.7)	(4.4)
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(10.0)	(11.7)	(3.9)
Risk-management
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	(9.9)	7.6	(4.9)
Run-off reinsurance lines
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	0.0	0.4	0.7
Other run-off lines
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	$ (20.1)	$ (3.4)	$ (1.7)