Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Direct Premiums Written			$ 800.1	$ 742.8
Net premiums earned	$ 767.1	$ 720.7	767.1	720.7
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			631.8	591.5
Net premiums earned			599.0	563.8
Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			99.1	89.1
Net premiums earned			99.5	92.0
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			40.9	36.2
Net premiums earned			38.7	33.8
Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			28.3	26.0
Net premiums earned			29.9	31.1
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			792.7	735.8
Net premiums earned	755.8	707.2	755.8	707.2
Property and Casualty Lines [Member] | Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			631.8	591.5
Net premiums earned			599.0	563.8
Property and Casualty Lines [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			99.1	89.1
Net premiums earned			99.5	92.0
Property and Casualty Lines [Member] | Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			40.9	36.2
Net premiums earned			38.7	33.8
Property and Casualty Lines [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			20.9	19.0
Net premiums earned			18.6	17.6
Other Segments [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			7.4	7.0
Net premiums earned	$ 11.3	$ 13.5	11.3	13.5
Other Segments [Member] | Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			0.0	0.0
Net premiums earned			0.0	0.0
Other Segments [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			0.0	0.0
Net premiums earned			0.0	0.0
Other Segments [Member] | Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			0.0	0.0
Net premiums earned			0.0	0.0
Other Segments [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written			7.4	7.0
Net premiums earned			$ 11.3	$ 13.5