Asbestos Related Illnesses and Environmental Impairment Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	$ 109.5	$ 113.0
Net asbestos-related illness and environmental impairment reserves	105.6	108.2
Reinsurance Segment
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	105.0	108.4
Net asbestos-related illness and environmental impairment reserves	101.1	103.6
Insurance Segment | CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	4.5	4.6
Net asbestos-related illness and environmental impairment reserves	$ 4.5	$ 4.6