Run-Off Lines - Results for Run-Off Lines (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ (10.9)	$ (4.4)	$ (13.3)
Total Run-off Lines	(25.1)	(14.1)	(31.0)
Reinsurance Assumed [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(1.3)	(1.0)	(8.3)
Other [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(9.6)	(3.4)	(5.0)
Risk Management [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Run-off Lines	(13.1)	(8.2)	(15.5)
Run-Off Reinsurance Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Run-off Lines	0.1	2.0	(1.5)
Other Run-Off Lines [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total Run-off Lines	$ (1.2)	$ (3.5)	$ (0.7)