Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 808.1	$ 789.8
Direct Premiums Written	869.0	814.2
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	627.0	613.3
Direct Premiums Written	674.0	639.0
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	107.8	105.7
Direct Premiums Written	115.3	103.8
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	44.9	41.3
Direct Premiums Written	49.7	43.0
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	28.4	29.5
Direct Premiums Written	30.0	28.4
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	800.3	780.8
Direct Premiums Written	862.6	808.5
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	627.0	613.3
Direct Premiums Written	674.0	639.0
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	107.8	105.7
Direct Premiums Written	115.3	103.8
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	44.9	41.3
Direct Premiums Written	49.7	43.0
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	20.6	20.5
Direct Premiums Written	23.6	22.7
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7.8	9.0
Direct Premiums Written	6.4	5.7
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	7.8	9.0
Direct Premiums Written	$ 6.4	$ 5.7