Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenues from external sources:
Revenue	$ 294.4	$ 285.3	$ 585.7	$ 570.6
Net Investment Income	20.5	19.7	38.1	36.6
Segment income before federal income tax:
Gaap Expense Adjustments	0.8	0.8	1.6	1.5
Interest expense on corporate debt	(1.3)	(3.8)	(2.6)	(5.5)
Income before federal income taxes	3.1	6.3	30.8	26.3
Operating Segments [Member]
Revenues from external sources:
Revenue	294.4	285.3	585.7	570.6
Segment income before federal income tax:
Income before federal income taxes	(3.0)	4.4	21.5	22.6
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	113.7	117.2	228.0	231.9
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	95.9	89.6	189.4	178.1
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	58.7	56.7	113.4	114.8
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	268.3	263.5	530.8	524.8
Segment income before federal income tax:
Income before federal income taxes	(28.7)	(17.3)	(32.6)	(23.0)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	20.5	19.7	38.1	36.6
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	25.8	21.2	54.1	44.8
Segment income before federal income tax:
Income before federal income taxes	25.8	21.2	54.1	44.8
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	5.3	1.5	16.0	8.2
Segment income before federal income tax:
Income before federal income taxes	5.3	1.5	16.0	8.2
Operating Segments [Member] | All Other Segments [Member]
Segment income before federal income tax:
Income before federal income taxes	(0.1)	0.5	0	0.8
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(10.7)	(6.8)	(4.6)	(5.5)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(5.9)	(4.5)	(15.3)	(9.8)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(12.1)	(6.0)	(12.7)	(7.7)
Corporate and Other [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0.3	0.6	0.8	1.0
Corporate and Other [Member] | Gaap Adjustments [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	8.9	7.2	14.2	11.8
Corporate and Other [Member] | Corporate Segment [Member]
Segment income before federal income tax:
Corporate Expenses	(1.5)	(1.5)	(2.3)	(2.6)
Corporate and Other [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Interest expense on corporate debt	1.3	3.8	2.6	5.5
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	(1.3)	(1.3)	(2.6)	(2.6)
Eliminate Intersegment Revenues [Member] | Intersegment [Member]
Revenues from external sources:
Revenue	1.3	1.3	2.6	2.6
External and Intersegment Sources [Member]
Revenues from external sources:
Revenue	295.7	286.6	588.3	573.2
Total Consolidated [Member]
Revenues from external sources:
Revenue	294.4	285.3	585.7	570.6
Segment Reconciling Items [Member]
Segment income before federal income tax:
Income before federal income taxes	$ 6.1	$ 1.9	$ 9.3	$ 3.7