Property and Casualty Premiums Written and Earned (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Ceded Credit Risk [Line Items]
Gross premiums written - direct	$ 1,231.4		$ 1,088.6		$ 1,080.5
Gross premiums written - assumed	2,991.5	[1]	52.4	[1]	23.5	[1]
Ceded premiums written	(499.0)	[1]	(366.3)	[1]	(367.8)	[1]
Net premiums written	3,723.9		774.7		736.2
Gross premiums earned - direct	1,161.5		1,069.1		1,131.7
Gross premiums earned - assumed	3,093.5	[1]	45.9	[1]	21.0	[1]
Ceded premiums earned	(522.0)	[1]	(367.4)	[1]	(384.6)	[1]
Net premiums earned	$ 3,733.0		$ 747.6		$ 768.1

[1] Amounts for 2012 include TransRe amounts from the Acquisition Date through December 31, 2012. See Note 2 for additional information on TransRe.