Asbestos Related Illnesses and Environmental Impairment Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	$ 117.8	$ 133.7
Net asbestos-related illness and environmental impairment reserves	113.1	130.2
Reinsurance Segment
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	113.0	128.1
Net asbestos-related illness and environmental impairment reserves	108.3	124.6
Insurance Segment | CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	4.8	5.6
Net asbestos-related illness and environmental impairment reserves	$ 4.8	$ 5.6