Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from external sources:
Revenue	$ 346.0	$ 358.4	$ 1,011.4	$ 1,059.2
Other Income	0.6	0.5	1.8	1.7
Net Investment Income	20.8	18.7	62.2	56.5
Realized Investment Gains (Losses)	17.8	20.2	18.2	43.8
Segment income before federal income tax:
Interest expense on corporate debt	(1.2)	(1.5)	(4.5)	(4.4)
Income before federal income taxes	41.0	(11.2)	45.4	(3.3)
Operating Segments [Member]
Revenues from external sources:
Revenue	346.0	358.4	1,011.4	1,059.2
Segment income before federal income tax:
Income before federal income taxes	42.3	(6.2)	58.3	10.0
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	173.0	144.1	494.2	430.6
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	115.2	114.3	345.7	341.8
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	18.6	60.6	89.3	184.8
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	306.8	319.0	929.2	957.2
Segment income before federal income tax:
Income before federal income taxes	3.6	(45.2)	(22.4)	(90.5)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income		18.7		56.5
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	38.6	38.9	80.4	100.3
Segment income before federal income tax:
Income before federal income taxes	38.6	38.9	80.4	100.3
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Segment income before federal income tax:
Income before federal income taxes		20.2	18.2	43.8
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue		0.5		1.7
Segment income before federal income tax:
Income before federal income taxes	0.1	0.1	0.3	0.2
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	6.9	(7.4)	(4.6)	(31.4)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(5.2)	1.3	(22.3)	(7.0)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	1.9	(39.1)	4.5	(52.1)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.6	1.6	4.8	3.0
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.6)	(1.6)	(4.8)	(3.0)
Segment income before federal income tax:
Corporate Expenses	(0.9)	(0.7)	(5.8)	(2.6)
Income before federal income taxes	(1.3)	(5.0)	(12.9)	(13.3)
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	0.8	(2.8)	(2.6)	(6.3)
Interest expense on corporate debt	1.2	1.5	4.5	4.4
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 347.6	$ 360.0	$ 1,016.2	$ 1,062.2