Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets
Unearned premium reserve reduction	$ 11.7	$ 11.5
Compensation accruals	9.6	9.5
Impaired securities	2.3	2.1
Other comprehensive income - net funded status of benefit plans	2.7	3.0
Discounting of unpaid claims and claim expense tax reserves	2.5	2.5
Intangibles	0.1	0.0
Postretirement benefits other than pensions	0.3	0.3
Total gross deferred tax assets	29.2	28.9
Deferred tax liabilities
Other comprehensive income - net unrealized gains on securities	101.1	124.7
Deferred policy acquisition costs	37.3	36.3
Life insurance future policy benefit reserve	30.7	26.7
Life insurance future policy benefit reserve (transitional rule)	8.5	10.7
Discounting of unpaid claims and claim expense tax reserves (transitional rule)	0.6	0.8
Investment related adjustments	37.3	34.2
Intangibles	0.0	0.2
Other, net	4.2	2.0
Total gross deferred tax liabilities	219.7	235.6
Net deferred tax liability	$ 190.5	$ 206.7