Run-Off Lines - Net Underwriting Results for Run-Off Lines (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ (13.3)	$ (11.0)	$ (9.1)
Total Run-off Lines	(31.0)	(19.6)	(10.9)
Reinsurance Assumed [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(8.3)	(3.8)	(3.6)
Other [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(5.0)	(7.2)	(5.5)
Risk Management [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Run-off Lines	(15.5)	(5.9)	0.9
Run-Off Reinsurance Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Run-off Lines	(1.5)	(1.9)	(1.1)
Other Run-Off Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Run-off Lines	$ (0.7)	$ (0.8)	$ (1.6)