Run-off Lines - Underwriting Losses for Run-Off Lines (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	$ (8.3)	$ (8.0)	$ (15.4)
Total underwriting loss - Run-off Lines	(14.2)	(15.9)	(25.7)
Reinsurance Assumed [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(4.4)	(3.9)	(8.7)
Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total Asbestos and Environmental	(3.8)	(4.1)	(6.7)
Risk Management [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	(4.9)	(2.6)	(8.8)
Run-Off Reinsurance Lines [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	0.7	0.0	(0.1)
Other Run-Off Lines [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total underwriting loss - Run-off Lines	$ (1.7)	$ (5.3)	$ (1.4)