Run-off Lines - Total Gross Reserves for Asbestos Exposure (Detail) - Asbestos and Environmental Claims - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	$ 17.7	$ 21.1	$ 23.4
Total asbestos reserves	40.6	46.7	47.9
Case reserves
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	2.9	3.2	3.0
Unallocated loss adjustment expense
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	0.5	0.5	0.5
Incurred but not reported
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	14.3	17.4	19.9
Assumed domestic
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	20.0	20.6	20.1
Assumed domestic | Case reserves
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	7.6	6.8	7.4
Assumed domestic | Unallocated loss adjustment expense
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	0.8	0.8	0.8
Assumed domestic | Incurred but not reported
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	11.6	13.0	11.9
Assumed London
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	2.9	5.0	4.4
Assumed London | Case reserves
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	2.8	2.4	2.1
Assumed London | Incurred but not reported
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	$ 0.1	$ 2.6	$ 2.3