Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Loss, LAE and unearned premium reserves, net	$ 150.8	$ 139.9
Employee benefit plans	6.0	7.6
Other	16.9	8.3
Total deferred tax assets	173.7	155.8
Investments, net	146.7	87.9
Deferred acquisition costs	100.3	98.1
Software capitalization	24.0	21.0
Other		0.6
Total deferred tax liabilities	271.0	207.6
Net deferred tax liability	$ (97.3)	$ (51.8)