Statutory Financial Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
United States Traditional Insurance Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory Net Income	$ 624.5	$ 642.9	$ 628.8
Statutory Combined Net Gain from Operations	649.8	664.0	645.7
Tailwind Re and Northwind Re Subsidiaries [Member]
Statutory Accounting Practices [Line Items]
Statutory Net Income	69.7	80.0	79.1
Statutory Combined Net Gain from Operations	$ 69.3	$ 80.4	$ 79.2