Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Deferred tax assets
Other comprehensive income - net unrealized losses on securities	$ 85.2	$ 82.1
Unearned premium reserve reduction	14.4	13.3
Compensation accruals	9.3	9.3
Impaired securities	2.3	2.0
Other comprehensive income - net funded status of benefit plans	1.9	2.0
Discounting of unpaid claims and claim expense tax reserves	2.9	2.7
Capital loss carryforward	1.2	0.8
Net operating loss carryforward	0.6	9.3
Intangibles	0.0	1.4
Postretirement benefits other than pensions	0.2	0.2
Total gross deferred tax assets	118.0	123.1
Deferred tax liabilities
Deferred policy acquisition costs	47.8	45.9
Life insurance future policy benefit reserve	52.7	46.4
Life insurance future policy benefit reserve (transitional rule)	2.1	4.3
Discounting of unpaid claims and claim expense tax reserves (transitional rule)	0.2	0.3
Investment related adjustments	39.3	44.8
Other comprehensive income - net reserve remeasurements	23.8	5.8
Other, net	7.9	9.0
Total gross deferred tax liabilities	173.8	156.5
Net deferred tax liability	$ 55.8	$ 33.4