Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues from external sources:
Revenue	$ 358.4	$ 352.8	$ 1,058.4	$ 1,040.5
Net Investment Income	18.7	17.6	56.5	54.1
Segment income before federal income tax:
Interest expense on corporate debt	(1.5)	(1.4)	(4.4)	(4.1)
Income before federal income taxes	(11.2)	7.4	(4.5)	(13.9)
Operating Segments [Member]
Revenues from external sources:
Revenue	358.4	352.8	1,058.4	1,040.5
Segment income before federal income tax:
Income before federal income taxes	(6.2)	7.5	9.3	(12.7)
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	144.1	144.5	430.3	433.8
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	114.3	116.0	341.8	356.3
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	60.6	65.3	184.3	178.0
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	319.0	325.8	956.4	968.1
Segment income before federal income tax:
Income before federal income taxes	(45.2)	(19.0)	(91.2)	(83.7)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	18.7	17.6	56.5	54.1
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	38.9	26.4	100.3	70.7
Segment income before federal income tax:
Income before federal income taxes	38.9	26.4	100.3	70.7
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	20.2	8.8	43.8	16.6
Segment income before federal income tax:
Income before federal income taxes	20.2	8.8	43.8	16.6
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0.5	0.6	1.7	1.7
Segment income before federal income tax:
Income before federal income taxes	0.1	0.1	0.2	0.3
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(7.4)	1.9	(31.7)	(21.7)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	1.3	(17.6)	(7.0)	(33.7)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(39.1)	(3.3)	(52.5)	(28.3)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.6	1.5	3.0	4.4
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.6)	(1.5)	(3.0)	(4.4)
Segment income before federal income tax:
Corporate Expenses	(0.7)	(1.1)	(2.6)	(2.9)
Income before federal income taxes	(5.0)	(0.1)	(13.8)	(1.2)
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	(2.8)	2.4	(6.8)	5.8
Interest expense on corporate debt	1.5	1.4	4.4	4.1
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 360.0	$ 354.3	$ 1,061.4	$ 1,044.9