Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Life									$ 379.7	$ 381.6	$ 374.1
Premiums Earned, Net, Accident and Health									161.7	149.4	144.9
Earned Premiums	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 561.4	$ 558.9	$ 553.7	$ 546.0	2,350.0	2,220.0	2,009.6
Personal Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									1,377.1	1,244.6	1,027.7
Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									264.8	271.9	286.3
Other Personal Lines [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									115.3	119.2	122.1
Commercial Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									$ 51.4	$ 53.3	$ 54.5