Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Net premiums earned	$ 779.3	$ 731.6	$ 1,546.4	$ 1,452.3
Direct Premiums Written	784.0	735.9	1,584.1	1,478.7
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	607.1	570.9	1,206.1	1,134.7
Direct Premiums Written	595.4	562.6	1,227.2	1,154.1
Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	102.0	93.8	201.5	185.8
Direct Premiums Written	116.3	105.1	215.4	194.2
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	39.9	35.5	78.6	69.3
Direct Premiums Written	42.4	38.5	83.3	74.7
Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	30.3	31.4	60.2	62.5
Direct Premiums Written	29.9	29.7	58.2	55.7
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	768.6	718.5	1,524.4	1,425.7
Direct Premiums Written	777.1	727.5	1,569.8	1,463.3
Property and Casualty Lines [Member] \| Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	607.1	570.9	1,206.1	1,134.7
Direct Premiums Written	595.4	562.6	1,227.2	1,154.1
Property and Casualty Lines [Member] \| Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	102.0	93.8	201.5	185.8
Direct Premiums Written	116.3	105.1	215.4	194.2
Property and Casualty Lines [Member] \| Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	39.9	35.5	78.6	69.3
Direct Premiums Written	42.4	38.5	83.3	74.7
Property and Casualty Lines [Member] \| Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	19.6	18.3	38.2	35.9
Direct Premiums Written	23.0	21.3	43.9	40.3
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	10.7	13.1	22.0	26.6
Direct Premiums Written	6.9	8.4	14.3	15.4
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	10.7	13.1	22.0	26.6
Direct Premiums Written	$ 6.9	$ 8.4	$ 14.3	$ 15.4