Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues from external sources:
Revenue	$ 214.8	$ 367.6
Other Income	0.6	0.6
Net Investment Income	18.9	19.4
Realized Investment Gains (Losses)	(135.2)	44.9
Segment income before federal income tax:
Interest expense on corporate debt	(1.2)	(1.2)
Income before federal income taxes	(144.0)	61.6
Operating Segments [Member]
Revenues from external sources:
Revenue	214.8	367.6
Segment income before federal income tax:
Income before federal income taxes	(146.3)	61.3
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	198.1	181.5
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	132.4	116.4
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	0.0	4.8
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	330.5	302.7
Segment income before federal income tax:
Income before federal income taxes	(30.1)	(3.1)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	18.9	19.4
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	(116.3)	64.3
Segment income before federal income tax:
Income before federal income taxes	(116.3)	64.3
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Segment income before federal income tax:
Income before federal income taxes	(135.2)	44.9
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue		0.6
Segment income before federal income tax:
Income before federal income taxes	0.1	0.1
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	0.3	4.2
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(30.0)	(5.9)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(0.4)	(1.4)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.6	1.6
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.6)	(1.6)
Segment income before federal income tax:
Corporate Expenses	(2.4)	(2.6)
Income before federal income taxes	2.3	0.3
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	5.9	4.1
Interest expense on corporate debt	1.2	1.2
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 216.4	$ 369.2