Segment Information (Identifiable Assets) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Identifiable Assets [Line Items]
Identifiable assets	$ 6,649.8	$ 6,378.3
Excess and Surplus Lines
Identifiable Assets [Line Items]
Identifiable assets	2,250.9	2,271.1
Commercial Specialty
Identifiable Assets [Line Items]
Identifiable assets	1,307.3	1,285.4
International Specialty
Identifiable Assets [Line Items]
Identifiable assets	736.7	694.7
Syndicate 1200 [Member]
Identifiable Assets [Line Items]
Identifiable assets	1,578.5	1,544.5
Run-Off Lines
Identifiable Assets [Line Items]
Identifiable assets	547.7	523.7
Corporate and Other
Identifiable Assets [Line Items]
Identifiable assets	$ 228.7	$ 58.9