Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Deferred tax assets
Other comprehensive income - net unrealized losses on securities	$ 50.0	$ 85.2
Unearned premium reserve reduction	15.1	14.4
Compensation accruals	12.7	9.3
Impaired securities	3.2	2.3
Other comprehensive income - net funded status of benefit plans	0.7	1.9
Discounting of unpaid claims and claim expense tax reserves	2.9	2.9
Capital loss carryforward	0.0	1.2
Net operating loss carryforward	0.0	0.6
Postretirement benefits other than pensions	0.2	0.2
Other comprehensive income - net reserve remeasurements	4.7	0.0
Total gross deferred tax assets	89.5	118.0
Deferred tax liabilities
Deferred policy acquisition costs	47.1	47.8
Life insurance future policy benefit reserve	81.5	52.7
Life insurance future policy benefit reserve (transitional rule)	0.0	2.1
Discounting of unpaid claims and claim expense tax reserves (transitional rule)	0.0	0.2
Investment related adjustments	15.4	37.7
Intangibles	1.2	1.6
Other comprehensive income - net reserve remeasurements	0.0	23.8
Other, net	9.8	7.9
Total gross deferred tax liabilities	155.0	173.8
Net deferred tax liability	$ 65.5	$ 55.8