Statutory Financial Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
United States Traditional Insurance Subsidiaries
Statutory Accounting Practices
Statutory Net Income	$ 653.7	$ 623.1	$ 584.5
Statutory Combined Net Gain from Operations	689.2	618.1	617.5
Statutory Combined Capital and Surplus	3,470.3	3,462.8
Captive Reinsurers
Statutory Accounting Practices
Statutory Net Income	(56.3)	(123.0)	13.3
Statutory Combined Net Gain from Operations	(54.3)	(123.8)	$ 13.6
Statutory Combined Capital and Surplus	$ 1,672.3	$ 1,668.3