Loss and Loss Adjustment Expense Including Amounts for Risks Relating to Asbestos Related Illnesses and Environmental Impairment (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Gross reserves	$ 606.8	$ 602.7
Net reserves	447.6	447.4
TransRe
Commitments and Contingencies [Line Items]
Gross reserves	597.9	593.5
Net reserves	438.8	438.3
CapSpecialty Incorporated
Commitments and Contingencies [Line Items]
Gross reserves	8.9	9.2
Net reserves	$ 8.8	$ 9.1