Loss and Loss Adjustment Expense Including Amounts for Risks Relating to Asbestos Related Illnesses and Environmental Impairment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Commitments and Contingencies [Line Items]
Gross reserves	$ 608.4	$ 594.2
Net reserves	450.7	442.2
TransRe
Commitments and Contingencies [Line Items]
Gross reserves	597.9	583.8
Net reserves	440.3	431.9
Capitol
Commitments and Contingencies [Line Items]
Gross reserves	10.5	10.4
Net reserves	$ 10.4	$ 10.3