Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Net premiums earned	$ 801.2	$ 790.9	$ 2,388.6	$ 2,337.3
Direct Premiums Written	835.1	817.0	2,450.0	2,401.1
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	619.8	614.0	1,850.3	1,820.1
Direct Premiums Written	633.7	630.1	1,872.8	1,857.3
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	108.3	105.5	322.3	307.0
Direct Premiums Written	125.8	116.0	352.3	331.4
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	43.6	41.2	127.3	119.8
Direct Premiums Written	45.6	41.9	133.7	125.2
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29.5	30.2	88.7	90.4
Direct Premiums Written	30.0	29.0	91.2	87.2
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	792.7	780.6	2,362.4	2,305.0
Direct Premiums Written	828.0	810.4	2,428.7	2,380.2
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	619.8	614.0	1,850.3	1,820.1
Direct Premiums Written	633.7	630.1	1,872.8	1,857.3
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	108.3	105.5	322.3	307.0
Direct Premiums Written	125.8	116.0	352.3	331.4
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	43.6	41.2	127.3	119.8
Direct Premiums Written	45.6	41.9	133.7	125.2
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	21.0	19.9	62.5	58.1
Direct Premiums Written	22.9	22.4	69.9	66.3
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	8.5	10.3	26.2	32.3
Direct Premiums Written	7.1	6.6	21.3	20.9
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	8.5	10.3	26.2	32.3
Direct Premiums Written	$ 7.1	$ 6.6	$ 21.3	$ 20.9