Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Loss, LAE and unearned premium reserves, net	$ 129.8	$ 115.7
Employee benefit plans	14.5	18.7
Tax credit carryforwards		23.1
Investments, net	4.1
Other	16.3	11.5
Deferred tax assets, gross	164.7	169.0
Less: Valuation allowance	0.2	0.2
Deferred tax assets, total	164.5	168.8
Deferred acquisition costs	95.0	90.2
Investments, net		41.8
Software capitalization	18.3	18.0
Other	0.6	1.8
Deferred tax liabilities, total	113.9	151.8
Net deferred tax asset	$ 50.6	$ 17.0