Asbestos Related Illnesses and Environmental Impairment Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	$ 113.0	$ 117.8
Net asbestos-related illness and environmental impairment reserves	108.2	113.1
Reinsurance Segment
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	108.4	113.0
Net asbestos-related illness and environmental impairment reserves	103.6	108.3
Insurance Segment | CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	4.6	4.8
Net asbestos-related illness and environmental impairment reserves	$ 4.6	$ 4.8