Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums, Life	$ 94.5	$ 95.8	$ 285.6	$ 285.7
Earned Premiums, Accident and Health	42.0	37.6	120.6	111.2
Total Earned Premiums	598.2	558.9	1,744.1	1,658.6
Personal Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums, Property and Casualty	353.0	313.8	1,012.8	927.4
Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums, Property and Casualty	66.7	68.5	199.6	204.2
Other Personal Property and Casualty Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums, Property and Casualty	28.9	29.9	87.0	89.9
Commercial Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums, Property and Casualty	$ 13.1	$ 13.3	$ 38.5	$ 40.2