Supplemental Information Concerning Property and Casualty Insurance Operations (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred policy acquisition costs	$ 489.5		$ 458.6		$ 309.7
Reserves for unpaid claims and claim adjustment expenses	6,197.0	[1]	5,760.3	[1]	3,277.7	[1]
Discount, if any, deducted from previous column		[2]		[2]		[2]
Unearned premiums	2,474.8	[1]	2,292.1	[1]	1,520.3	[1]
Earned premiums	4,239.1		3,598.6		2,841.0
Net investment income	276.6		258.2		247.2
Claims and claim adjustment expenses incurred Current year	2,990.2		2,654.1		1,967.4
Claims and claim adjustment expenses incurred Prior years	(15.8)		(103.3)		(111.1)
Amortization of deferred policy acquisition costs	938.1		778.9		600.8
Paid claims and claim adjustment expenses	2,714.1		2,492.7		1,817.3
Premiums written	$ 4,368.4		$ 3,593.4		$ 3,048.0

[1] Reserves for unpaid claims and claim adjustment expenses are shown gross of $2,074.3 million, $1,931.8 million and $1,115.5 million of reinsurance recoverable on unpaid losses in 2012, 2011 and 2010, respectively. Unearned premiums are shown gross of prepaid premiums of $275.4 million, $234.9 million and $77.0 million in 2012, 2011 and 2010, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.

[2] The Company does not use discounting techniques.