Loss and Loss Adjustment Expense Including Amounts for Risks Relating to Asbestos Related Illnesses and Environmental Impairment (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	$ 183.6	$ 602.7
Net asbestos-related illness and environmental impairment reserves	177.0	447.4
TransRe
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	174.9	593.5
Net asbestos-related illness and environmental impairment reserves	168.4	438.3
CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross asbestos-related illness and environmental impairment reserves	8.7	9.2
Net asbestos-related illness and environmental impairment reserves	$ 8.6	$ 9.1