Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Earned premiums, net	$ 472.0	$ 437.6	$ 212.4
Net investment income, affiliates	4.9	4.9	4.9
Other income, affiliates	2.3	2.1	1.9
Losses and loss expenses, net affiliates	383.8	293.3	250.8
Net Acquisition and Operating Expenses	285.8	248.6	156.9
Interest Expense Debt To Affiliates	$ 0.8	$ 0.7	$ 0.7