Run-off Lines - Total Gross Reserves for Asbestos Exposure (Detail) - Asbestos and Environmental Claims - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	$ 21.1	$ 23.4	$ 23.8
Total asbestos reserves	46.7	47.9	48.8
Case reserves
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	3.2	3.0	3.1
Unallocated loss adjustment expense ("ULAE")
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 ("IBNR")
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total direct written reserves	17.4	19.9	20.2
Assumed domestic
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	20.6	20.1	22.0
Assumed domestic | Case reserves
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	6.8	7.4	8.4
Assumed domestic | Unallocated loss adjustment expense ("ULAE")
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 ("IBNR")
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	13.0	11.9	12.8
Assumed London
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	5.0	4.4	3.0
Assumed London | Case reserves
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	2.4	2.1	1.4
Assumed London | Incurred but not reported ("IBNR")
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total assumed reserves	$ 2.6	$ 2.3	$ 1.6