Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues from external sources:
Revenue	$ 355.7	$ 348.5	$ 700.0	$ 687.7
Net Investment Income	19.1	19.1	37.8	36.5
Segment income before federal income tax:
Interest expense on corporate debt	(1.5)	(1.4)	(2.9)	(2.7)
Income before federal income taxes	11.9	(25.1)	6.7	(21.3)
Operating Segments [Member]
Revenues from external sources:
Revenue	355.7	348.5	700.0	687.7
Segment income before federal income tax:
Income before federal income taxes	13.1	(27.3)	15.6	(20.2)
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	152.5	153.1	305.2	306.6
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	104.9	111.6	208.5	223.0
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	62.7	57.7	123.7	112.7
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	320.1	322.4	637.4	642.3
Segment income before federal income tax:
Income before federal income taxes	(21.9)	(53.0)	(46.0)	(64.7)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	19.1	19.1	37.8	36.5
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	34.9	25.6	61.4	44.3
Segment income before federal income tax:
Income before federal income taxes	34.9	25.6	61.4	44.3
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	15.8	6.5	23.6	7.8
Segment income before federal income tax:
Income before federal income taxes	15.8	6.5	23.6	7.8
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0.7	0.5	1.2	1.1
Segment income before federal income tax:
Income before federal income taxes	0.1	0.1	0.2	0.2
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(17.9)	(24.9)	(28.5)	(20.5)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	0.8	(11.8)	(4.1)	(19.2)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(4.8)	(16.3)	(13.4)	(25.0)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.6	1.5	3.0	2.9
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.6)	(1.5)	(3.0)	(2.9)
Segment income before federal income tax:
Corporate Expenses	(1.1)	(1.0)	(2.0)	(1.8)
Income before federal income taxes	(1.2)	2.2	(8.9)	(1.1)
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	1.4	4.6	(4.0)	3.4
Interest expense on corporate debt	1.5	1.4	2.9	2.7
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 357.3	$ 350.0	$ 703.0	$ 690.6