Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Net premiums earned	$ 790.9	$ 745.5	$ 2,337.3	$ 2,197.8
Direct Premiums Written	817.0	785.4	2,401.1	2,264.1
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	614.0	581.3	1,820.1	1,716.0
Direct Premiums Written	630.1	609.0	1,857.3	1,763.1
Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	105.5	95.8	307.0	281.6
Direct Premiums Written	116.0	106.6	331.4	300.8
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	41.2	37.0	119.8	106.3
Direct Premiums Written	41.9	40.1	125.2	114.8
Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	30.2	31.4	90.4	93.9
Direct Premiums Written	29.0	29.7	87.2	85.4
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	780.6	732.9	2,305.0	2,158.6
Direct Premiums Written	810.4	778.0	2,380.2	2,241.3
Property and Casualty Lines [Member] | Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	614.0	581.3	1,820.1	1,716.0
Direct Premiums Written	630.1	609.0	1,857.3	1,763.1
Property and Casualty Lines [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	105.5	95.8	307.0	281.6
Direct Premiums Written	116.0	106.6	331.4	300.8
Property and Casualty Lines [Member] | Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	41.2	37.0	119.8	106.3
Direct Premiums Written	41.9	40.1	125.2	114.8
Property and Casualty Lines [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	19.9	18.8	58.1	54.7
Direct Premiums Written	22.4	22.3	66.3	62.6
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10.3	12.6	32.3	39.2
Direct Premiums Written	6.6	7.4	20.9	22.8
Other Segments [Member] | Property and Casualty, Personal Insurance Product 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] | Property Insurance Product 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] | Property and Casualty, Commercial Insurance Product 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 Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10.3	12.6	32.3	39.2
Direct Premiums Written	$ 6.6	$ 7.4	$ 20.9	$ 22.8