Schedule VI. Supplemental Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Line Items]
Unearned premiums	$ 304.7	$ 299.1
Net investment income	72.7	76.3	$ 88.1
Deferred policy acquisition costs	43.7	43.2
Unpaid losses and loss adjustment expenses	1,981.2	2,069.4	2,192.8	$ 2,207.9
Prior years	(39.8)	(81.6)	(77.5)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	394.8	402.2	421.8
Net premiums earned	574.4	615.3	695.8
Current year	366.5	395.9	456.1
Premiums Written, Net	583.1	574.9	691.5
Losses and loss adjustment expenses	315.2	302.4	365.9
Employers Segment [Member]
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Line Items]
Unearned premiums	303.9	298.9	337.0
Net investment income	69.3	72.1	84.1
Deferred policy acquisition cost, amortization expense	92.2	97.5	107.7
Deferred policy acquisition costs	43.7	43.2	47.9
Unpaid losses and loss adjustment expenses	1,938.0	2,024.5	2,145.2
Prior years	(39.8)	(81.6)	(77.5)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	394.8	402.2	421.8
Net premiums earned	573.7	615.1	695.8
Current year	366.0	395.8	456.1
Premiums Written, Net	581.6	574.6	691.4
Losses and loss adjustment expenses	326.2	314.2	378.6
Cerity Segment [Member]
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Line Items]
Unearned premiums	0.8	0.2	0.1
Net investment income	2.8	3.1	0.3
Deferred policy acquisition cost, amortization expense	0.0	0.0	0.0
Deferred policy acquisition costs	0.0	0.0	0.0
Unpaid losses and loss adjustment expenses	0.6	0.1	0.0
Prior years	0.0	0.0	0.0
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	0.0	0.0	0.0
Net premiums earned	0.7	0.2	0.0
Current year	0.5	0.1	0.0
Premiums Written, Net	1.5	0.3	0.1
Losses and loss adjustment expenses	0.5	0.1	0.0
Corporate Segment [Member]
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Line Items]
Unearned premiums	0.0	0.0	0.0
Net investment income	0.6	1.1	3.7
Deferred policy acquisition cost, amortization expense	0.0	0.0	0.0
Deferred policy acquisition costs	0.0	0.0	0.0
Unpaid losses and loss adjustment expenses	42.6	44.8	47.6
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	(11.5)	(11.9)	(12.7)
Net premiums earned	0.0	0.0	0.0
Current year	0.0	0.0	0.0
Premiums Written, Net	0.0	0.0	0.0
Losses and loss adjustment expenses	$ (11.5)	$ (11.9)	$ (12.7)