Asbestos Related Illnesses and Environmental Impairment Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	$ 158.4	$ 172.0
Net asbestos-related illness and environmental impairment reserves	154.0	166.3
Reinsurance Segment
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	152.4	165.7
Net asbestos-related illness and environmental impairment reserves	148.0	160.0
Insurance Segment | CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	6.0	6.3
Net asbestos-related illness and environmental impairment reserves	$ 6.0	$ 6.3