Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Funds held by or deposited with reinsurers	$ 3.1	$ 3.2
Restricted cash and cash equivalents	0.2	0.2
Premiums receivable from policyholders for final audit	13.8	2.3
Premium Receivable, Allowance for Credit Loss	10.3	10.8	$ 4.6
Write-offs, net of recoveries of amounts previously written off	2.5	6.8	10.5
Premium Deficiency	0.0	0.0	0.0
Unpaid losses	$ 476.3	496.6
LPT - loss expense as a percentange of losses paid, for management of LPT claims	7.00%
LPT favorable/ unfavorable difference, percentage	30.00%
LPT actual amounts paid versus expected amounts, period	every five years
LPT actual amounts paid versus expected amounts, expiration period	25
Collateralized by cash or letter of credit	$ 1.8
LPT collateral held in trust account	432.5	369.1
Goodwill and other intangible asset impairment	0.0	0.0
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Intangible assets, gross carrying amount	23.0	23.0
Intangible assets, accumulated amortization	(9.4)	(9.4)
Intangible assets, net	13.6	13.6
Amortization of intangible assets	0.0	0.0
Employers Segment [Member]
Deferred policy acquisition cost, amortization expense	92.2	97.5	$ 107.7
Funds Held in Trust, Reinsurance Agreement [Domain]
Restricted cash and cash equivalents	0.2
Licensing Agreements [Member]
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Intangible assets, gross carrying amount	13.5	13.5
Intangible assets, accumulated amortization	0.0	0.0
Intangible assets, net	13.5	13.5
Service Agreements [Member]
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Intangible assets, gross carrying amount	9.4	9.4
Intangible assets, accumulated amortization	(9.4)	(9.4)
Intangible assets, net	0.0	0.0
Other Intangible Assets [Member]
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Intangible assets, gross carrying amount	0.1
Other Intangible Assets [Member]
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Intangible assets, accumulated amortization	0.0	0.0
Finite-Lived Intangible Assets, Net	$ 0.1	0.1
Finite-Lived Intangible Assets, Gross		0.1
Electronic Data Processing Equipment, Software, Furniture and Equipment and Automobiles [Member]
Finite-Lived intangible assets, amortization method	straight-line method over three to seven years
Leasehold improvements
Finite-Lived intangible assets, amortization method	amortized over three to eight years
LPT Agreement [Member]
Cash and cash equivalents at fair value		$ 19.1