Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Net premiums earned	$ 789.8	$ 767.1
Direct Premiums Written	814.2	800.1
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	613.3	599.0
Direct Premiums Written	639.0	631.8
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	105.7	99.5
Direct Premiums Written	103.8	99.1
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	41.3	38.7
Direct Premiums Written	43.0	40.9
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29.5	29.9
Direct Premiums Written	28.4	28.3
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	780.8	755.8
Direct Premiums Written	808.5	792.7
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	613.3	599.0
Direct Premiums Written	639.0	631.8
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	105.7	99.5
Direct Premiums Written	103.8	99.1
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	41.3	38.7
Direct Premiums Written	43.0	40.9
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	20.5	18.6
Direct Premiums Written	22.7	20.9
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	9.0	11.3
Direct Premiums Written	5.7	7.4
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0.0	0.0
Direct Premiums Written	0.0	0.0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0.0	0.0
Direct Premiums Written	0.0	0.0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0.0	0.0
Direct Premiums Written	0.0	0.0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	9.0	11.3
Direct Premiums Written	$ 5.7	$ 7.4