Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues from external sources:
Revenue	$ 291.3	$ 292.7	$ 877.0	$ 863.3
Net Investment Income	18.9	18.8	57.0	55.4
Segment income before federal income tax:
Interest expense on corporate debt	(1.4)	(1.6)	(4.0)	(7.1)
Income before federal income taxes	13.1	18.7	43.9	45.0
Operating Segments [Member]
Revenues from external sources:
Revenue	291.3	292.7	877.0	863.3
Segment income before federal income tax:
Income before federal income taxes	10.3	22.3	31.8	44.8
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	112.3	116.4	340.3	348.3
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	96.0	92.9	285.4	271.0
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	61.9	56.7	175.3	171.5
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	270.2	266.0	801.0	790.8
Segment income before federal income tax:
Income before federal income taxes	(10.2)	(4.0)	(42.8)	(27.1)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	18.9	18.8	57.0	55.4
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	20.5	26.3	74.6	71.1
Segment income before federal income tax:
Income before federal income taxes	20.5	26.3	74.6	71.1
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	1.6	7.5	17.6	15.7
Segment income before federal income tax:
Income before federal income taxes	1.6	7.5	17.6	15.7
Operating Segments [Member] | All Other Segments [Member]
Segment income before federal income tax:
Income before federal income taxes	0	0	0	0.8
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	4.5	(6.0)	(0.1)	(11.5)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(3.3)	(1.1)	(18.6)	(11.0)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(11.4)	3.1	(24.1)	(4.6)
Corporate and Other [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0.6	0.4	1.4	1.4
Corporate and Other [Member] | Gaap Adjustments [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	4.2	(0.4)	18.4	11.4
Corporate and Other [Member] | Corporate Segment [Member]
Segment income before federal income tax:
Corporate Expenses	0	(1.6)	(2.3)	(4.1)
Corporate and Other [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Interest expense on corporate debt	1.4	1.6	4.0	7.1
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	(1.3)	(1.4)	(3.9)	(4.0)
Eliminate Intersegment Revenues [Member] | Intersegment [Member]
Revenues from external sources:
Revenue	1.3	1.4	3.9	4.0
External and Intersegment Sources [Member]
Revenues from external sources:
Revenue	292.6	294.1	880.9	867.3
Total Consolidated [Member]
Revenues from external sources:
Revenue	291.3	292.7	877.0	863.3
Segment Reconciling Items [Member]
Segment income before federal income tax:
Income before federal income taxes	$ 2.8	$ (3.6)	$ 12.1	$ 0.2