Statutory Financial Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
United States Traditional Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory Net Income	$ 642.9	$ 628.8	$ 639.2
Statutory Combined Net Gain from Operations	664.0	645.7	741.2
Tailwind Re and Northwind Re Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory Net Income	80.0	79.1	87.2
Statutory Combined Net Gain from Operations	$ 80.4	$ 79.2	$ 87.2