Loss and Loss Adjustment Expense Including Amounts for Risks Relating to Asbestos Related Illnesses and Environmental Impairment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Commitments and Contingencies [Line Items]
Gross reserves	$ 610.3	$ 594.2
Net reserves	449.8	442.2
TransRe
Commitments and Contingencies [Line Items]
Gross reserves	600.8	583.8
Net reserves	440.3	431.9
CapSpecialty
Commitments and Contingencies [Line Items]
Gross reserves	9.5	10.4
Net reserves	$ 9.5	$ 10.3