Reserves For Losses And Loss Adjustment Expenses (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Paid loss recoverables excluded from reinsurance recoverables on unpaid losses and LAE	$ 106.5	$ 148.5
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	(17.8)	1.6
Unfavorable Development [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	3.8	1.4
Workers Compensation [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	7.7	4.1
Short-Tail Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	2.4
International Specialty
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	4.5	3.6
Short-Tail Non-Catastrophe Losses [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	5.0	3.9
Long-Tail Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	0.3
General Liability [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	24.9	12.1
Automobile Liability Lines of Business [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	2.4
Property Lines of Business [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	3.1	1.2
Commercial Specialty
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	18.6	4.7
Legacy Company Claims [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		3.7
Development [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		1.1
Professional and General Liability [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		10.1
Run-Off Lines
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	6.0	3.5
Asbestos and Environmental Lines [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	5.5	11.7
Reinsurance Recoverable [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		8.2
Environmental Losses [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		3.5
Excess and Surplus Lines
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	(33.5)	17.8
Excess and Surplus Lines | General and Products Liability Lines of Business [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	(23.6)
Excess and Surplus Lines | General Liability [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		13.2
Excess and Surplus Lines | Automobile Liability Lines of Business [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years		3.4
Short-Tail Catastrophe Losses [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	0.8
Syndicate 1200 Segment [Member]
Schedule Of Reserves For Losses And Loss Adjustment Expenses [Line Items]
Liability for unpaid claims and claims adjustment expense, incurred claims, prior years	$ 4.4	$ 14.8