Valuation and Qualifying Accounts (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	$ 18.7		$ 5.9		$ 6.4
Charged to costs and expenses	8.5		12.3		7.8
Charged to other accounts	0.9	[1]	11.9	[1]
Deductions	(8.4)		(11.4)		(8.3)
Balance at end of period	19.7		18.7		5.9
Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	2.3		3.6		3.9
Charged to costs and expenses	8.1		10.1		7.8
Deductions	(7.6)		(11.4)		(8.1)
Balance at end of period	2.8		2.3		3.6
Allowance for uncollectible reinsurance recoverables
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	16.4		2.3		2.5
Charged to costs and expenses	0.4		2.2
Charged to other accounts	0.9	[1]	11.9	[1]
Deductions	(0.8)				(0.2)
Balance at end of period	$ 16.9		$ 16.4		$ 2.3

[1] Amount charged to other accounts includes foreign exchange gains and losses and in 2011 also includes the allowance for uncollectible reinsurance recoverables acquired from Chaucer Holdings plc on July 1, 2011.