Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets
Other comprehensive income - net unrealized losses on securities	$ 105.8	$ 0.0
Unearned premium reserve reduction	10.1	11.7
Compensation accruals	8.4	9.6
Impaired securities	2.0	2.3
Other comprehensive income - net funded status of benefit plans	2.3	2.7
Discounting of unpaid claims and claim expense tax reserves	2.8	2.5
Net operating loss carryforward	3.6	0.0
Intangibles	0.1	0.1
Postretirement benefits other than pensions	0.2	0.3
Total gross deferred tax assets	135.3	29.2
Deferred tax liabilities
Other comprehensive income - net unrealized gains on securities	0.0	101.1
Deferred policy acquisition costs	73.0	37.3
Life insurance future policy benefit reserve	30.9	30.7
Life insurance future policy benefit reserve (transitional rule)	6.4	8.5
Discounting of unpaid claims and claim expense tax reserves (transitional rule)	0.5	0.6
Investment related adjustments	29.9	37.3
Other, net	0.8	4.2
Total gross deferred tax liabilities	141.5	219.7
Net deferred tax liability	$ 6.2	$ 190.5