Acquisition of Business Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Apr. 30, 2015
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net			$ 71.0
Business Combination, Indemnification Assets, Amount as of Acquisition Date			5.4
Business Combination, Indemnification Asset	$ 16.0	$ 15.9
Investments [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			187.0
Cash [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			13.4
Premiums Receivable [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			44.4
Receivables Other Than Premium Receivables [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			52.8
Finite-Lived Intangible Assets [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 19.3	$ 22.0	32.6
Goodwill [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			11.2
Current Income Tax Asset [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			1.4
Other Assets Acquired [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets			5.9
Property and Casualty Insurance Reserves [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			(155.8)
Unearned Premiums [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			(85.6)
Liabilities for Income Taxes [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			(1.5)
Other Liabilities [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities			(34.8)
General Indemnification [Member]
Business Acquisition [Line Items]
Business Combination, Indemnification Assets, Range of Outcomes, Value, High			12.5
Employee Related Matters [Member]
Business Acquisition [Line Items]
Business Combination, Indemnification Assets, Range of Outcomes, Value, High			$ 5.0