Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Life									$ 381.6	$ 374.1	$ 387.6
Premiums Earned, Net, Accident and Health									149.4	144.9	148.6
Earned Premiums	$ 561.4	$ 558.9	$ 553.7	$ 546.0	$ 541.5	$ 536.7	$ 500.1	$ 431.3	2,220.0	2,009.6	1,862.2
Personal Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									1,244.6	1,027.7	831.4
Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									271.9	286.3	312.4
Other Personal Lines [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									119.2	122.1	127.4
Commercial Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Premiums Earned, Net, Property and Casualty									$ 53.3	$ 54.5	$ 54.8