Asbestos Related Illnesses and Environmental Impairment Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	$ 172.0	$ 183.6
Net asbestos-related illness and environmental impairment reserves	166.3	177.0
Reinsurance Segment
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	165.7	174.9
Net asbestos-related illness and environmental impairment reserves	160.0	168.4
Insurance Segment | CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	6.3	8.7
Net asbestos-related illness and environmental impairment reserves	$ 6.3	$ 8.6