Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Gross loss and LAE reserves, beginning of year	$ 5,760.3	$ 3,277.7	$ 3,153.9
Reinsurance recoverable on unpaid losses, beginning of year	1,931.8	1,115.5	1,060.2
Net loss and LAE reserves, beginning of year	3,828.5	2,162.2	2,093.7
Net incurred losses and LAE in respect of losses occurring in Current year	2,990.2	2,654.1	1,967.4
Net incurred losses and LAE in respect of losses occurring in prior year	(15.8)	(103.3)	(111.1)
Total incurred losses and LAE	2,974.4	2,550.8	1,856.3
Net payments of losses and LAE in respect of losses occurring in Current year	1,317.6	1,482.4	1,078.7
Net payments of losses and LAE in respect of losses occurring in Prior years	1,396.5	1,010.3	738.6
Total payments	2,714.1	2,492.7	1,817.3
Effect of foreign exchange rates	33.9	(22.8)
Net reserve for losses and LAE, end of year	4,122.7	3,828.5	2,162.2
Reinsurance recoverable on unpaid losses, end of year	2,074.3	1,931.8	1,115.5
Gross reserve for losses and LAE, end of year	6,197.0	5,760.3	3,277.7
Chaucer Business
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Purchase		1,631.0
Campania Holding Company, Inc.
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Purchase			$ 29.5