Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Document Period End Date			Jun. 30, 2017
Net premiums earned	$ 797.7	$ 779.3	$ 1,587.4	$ 1,546.4
Direct Premiums Written	800.6	784.0	1,614.9	1,584.1
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	617.3	607.1	1,230.5	1,206.1
Direct Premiums Written	600.1	595.4	1,239.1	1,227.2
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	108.3	102.0	214.0	201.5
Direct Premiums Written	122.7	116.3	226.5	215.4
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	42.4	39.9	83.7	78.6
Direct Premiums Written	45.1	42.4	88.1	83.3
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29.7	30.3	59.2	60.2
Direct Premiums Written	32.7	29.9	61.2	58.2
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	789.0	768.6	1,569.7	1,524.4
Direct Premiums Written	792.1	777.1	1,600.7	1,569.8
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	617.3	607.1	1,230.5	1,206.1
Direct Premiums Written	600.1	595.4	1,239.1	1,227.2
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	108.3	102.0	214.0	201.5
Direct Premiums Written	122.7	116.3	226.5	215.4
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	42.4	39.9	83.7	78.6
Direct Premiums Written	45.1	42.4	88.1	83.3
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	21.0	19.6	41.5	38.2
Direct Premiums Written	24.2	23.0	47.0	43.9
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	8.7	10.7	17.7	22.0
Direct Premiums Written	8.5	6.9	14.2	14.3
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Direct Premiums Written	0.0	0.0	0.0	0.0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Direct Premiums Written	0.0	0.0	0.0	0.0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Direct Premiums Written	0.0	0.0	0.0	0.0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	8.7	10.7	17.7	22.0
Direct Premiums Written	$ 8.5	$ 6.9	$ 14.2	$ 14.3