Schedule of Effects of Reinsurance (Parenthetical) (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 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
Information by reinsurer of credit risk arising from a reinsurance arrangement.
Reinsurance [Line Items]
Property and casualty losses and LAE, Assumed			96.0
Property and casualty losses and LAE, Ceded			$ 54.2

[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.