Income Taxes (Deferred Tax Components) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Decrease in valuation allowance	$ (4.7)
Deferred tax assets:
Reserve for losses and loss expenses	17.7	$ 22.2
Equity compensation	16.0	14.2
Unearned premium	17.1	18.1
Deferred acquisition costs	11.9	10.5
Mark-to-market losses	8.6	0.0
Net loss carryforward	21.0	27.3
Total deferred tax assets	92.3	92.3
Deferred tax liabilities:
Intangible assets	(21.9)	(23.3)
Mark-to-market gains	0.0	(3.0)
Depreciation	(4.4)	(6.4)
Market discount on bonds	(2.0)	(1.2)
Other	(1.2)	(5.2)
Total deferred tax liabilities	(29.5)	(39.1)
Net deferred taxes before valuation allowance	62.8	53.2
Valuation allowance	(24.1)	(28.8)
Net deferred tax assets	$ 38.7	$ 24.4