Schedule of Effects of Reinsurance (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Reinsurance [Line Items]
Property and casualty premiums written, Direct	$ 4,515.1		$ 3,830.9		$ 3,087.9
Property and casualty premiums written, Assumed	689.1	[1]	228.3	[1]	270.6	[1]
Property and casualty premiums written, Ceded	(835.8)		(465.8)		(310.5)
Property and casualty premiums written, Net	4,368.4		3,593.4		3,048.0
Property and casualty premiums earned, Direct	4,352.9		3,695.2		2,970.6
Property and casualty premiums earned, Assumed	687.0	[1]	442.6	[1]	174.8	[1]
Property and casualty premiums earned, Ceded	(800.8)		(539.2)		(304.4)
Property and casualty premiums earned, Net	4,239.1		3,598.6		2,841.0
Percentage of assumed to net premiums earned	16.21%		12.30%		6.15%
Property and casualty losses and LAE, Direct	3,251.4		2,723.5		2,001.8
Property and casualty losses and LAE, Assumed	311.4	[1]	249.4	[1]	96.9	[1]
Property and casualty losses and LAE, Ceded	(588.4)		(422.1)		(242.4)
Net losses and LAE	$ 2,974.4		$ 2,550.8		$ 1,856.3

[1] Assumed reinsurance activity in 2012 and 2011 primarily related to our Chaucer segment. In addition, 2011 assumed premiums earned and assumed losses and LAE included $96.0 million and $54.2 million, respectively, related to the 2010 OneBeacon renewal rights transaction. Assumed reinsurance activity in 2010 primarily related to the OneBeacon renewal rights transaction.