Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Earned premiums, net	$ 458.0	$ 472.0	$ 437.6
Net investment income, affiliates	4.9	4.9	4.9
Other income, affiliates	2.3	2.3	2.1
Losses and loss expenses, net affiliates	379.6	383.8	293.3
Net Acquisition and Operating Expenses	270.7	285.8	248.6
Interest Expense Debt To Affiliates	$ 0.8	$ 0.8	$ 0.7