Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statutory federal tax rate	21.00%	21.00%		35.00%
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current federal tax expense (benefit)	$ 20.2	$ 27.6	$ 26.3
Current state and local tax expense (benefit)	0.8	0.7	1.8
Current income tax expense (benefit)	21.0	28.3	28.1
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Routine/other	6.7	(0.4)	8.6
Deferred income tax expense (benefit)	6.7	(0.4)	8.6
Income tax expense	27.7	27.9	36.7
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Expense computed at statutory rate	31.0	31.0	40.7
Dividends received deduction and tax-exempt interest	(1.7)	(1.9)	(2.4)
LPT deferred gain amortization	(1.9)	(2.2)	(2.3)
Stock based compensation	0.1	(0.2)	(0.9)
LPT reserve adjustment	(0.5)	(0.3)	(0.4)
Income tax reconciliation, other	0.7	1.5	2.0
Net income	119.3	119.8	157.1
Unrecognized tax benefits	0.0	0.0	0.0
Income Taxes Paid, Net	28.2	18.5	37.8
Deferred Tax Liabilities [Abstract]
Unrealized capital gains, net	43.5	50.7
Deferred policy acquisition cost	9.4	9.2
Intangible assets	1.6	1.6
Deferred Tax Liabilities, Leasing Arrangements	3.0	3.6
Other deferred tax liabilities	6.7	8.1
Deferred income taxes, net	(7.7)	(15.5)
Deferred Tax Assets [Abstract]
Loss reserves discounting for tax reporting	30.0	30.3
Unearned premiums	11.9	11.5
Allowance for bad debt	2.3	2.4
Stock based compensation	2.5	3.4
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	4.1	3.5
Accrued liabilities	3.5	4.2
Other deferred tax assets	2.2	2.4
Deferred Tax Liabilities, Gross	64.2	73.2
Total deferred tax assets	56.5	57.7
Change Due to Estimated Reserves Ceded Under the LPT Agreement [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Net income	2.6	1.2	1.8
Change to Contingent Profit Commission [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Net income	$ 0.5	$ 0.2	$ 0.2