Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues from external sources:
Revenue	$ 391.6	$ 334.5	$ 1,041.2	$ 1,042.2
Other Income	0.5	0.6	1.6	1.6
Net Investment Income	17.9	19.3	54.5	60.4
Realized Investment Gains (Losses)	20.0	(5.0)	(39.3)	50.2
Segment income before federal income tax:
Interest expense on corporate debt	(1.1)	(1.2)	(3.6)	(3.7)
Income before federal income taxes	14.8	12.7	(87.6)	66.7
Operating Segments [Member]
Revenues from external sources:
Revenue	391.6	334.5	1,041.2	1,042.2
Segment income before federal income tax:
Income before federal income taxes	11.2	8.6	(100.6)	60.4
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	208.8	192.7	609.7	562.5
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	144.4	127.0	414.7	361.4
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	0.0	(0.1)	0.0	6.1
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	353.2	319.6	1,024.4	930.0
Segment income before federal income tax:
Income before federal income taxes	(26.7)	(5.7)	(115.9)	(50.4)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	17.9	19.3	54.5	60.4
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	37.9	14.3	15.2	110.6
Segment income before federal income tax:
Income before federal income taxes	37.9	14.3	15.2	110.6
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Realized Investment Gains (Losses)		(5.0)		50.2
Segment income before federal income tax:
Income before federal income taxes	20.0	(5.0)	(39.3)	50.2
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue		0.6		1.6
Segment income before federal income tax:
Income before federal income taxes	0.0	0.0	0.1	0.2
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(30.7)	(6.3)	(70.8)	(32.0)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	14.4	1.2	(28.0)	(17.1)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(10.4)	(0.6)	(17.1)	(1.3)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.5	1.7	4.6	4.9
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.5)	(1.7)	(4.6)	(4.9)
Segment income before federal income tax:
Corporate Expenses	(0.9)	(1.9)	(3.6)	(7.3)
Income before federal income taxes	3.6	4.1	13.0	6.3
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	5.6	7.2	20.2	17.3
Interest expense on corporate debt	1.1	1.2	3.6	3.7
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 393.1	$ 336.2	$ 1,045.8	$ 1,047.1