Run-Off Lines - Net Underwriting Results for Run-Off Lines (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Underwriting Results For Run-Off Lines [Line Items]
Total Asbestos and Environmental	$ (9.1)	$ (11.7)	$ (9.5)
Total Run-off Lines	(10.9)	(16.8)	(10.6)
Reinsurance Assumed [Member]
Net Underwriting Results For Run-Off Lines [Line Items]
Total Asbestos and Environmental	(3.6)	(6.7)	(9.5)
Other [Member]
Net Underwriting Results For Run-Off Lines [Line Items]
Total Asbestos and Environmental	(5.5)	(5.0)	0
Risk Management [Member]
Net Underwriting Results For Run-Off Lines [Line Items]
Total Run-off Lines	0.9	2.6	(1.8)
Run-off reinsurance lines [Member]
Net Underwriting Results For Run-Off Lines [Line Items]
Total Run-off Lines	(1.1)	(5.5)	0.7
Other Run-Off Lines [Member]
Net Underwriting Results For Run-Off Lines [Line Items]
Total Run-off Lines	$ (1.6)	$ (2.2)	$ 0