Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Premiums	$ 1,508.5	$ 1,448.6
Net investment income	106.1	89.7
Fees and other income	6.4	7.3
Total operating revenues	1,621.0	1,545.6
Net realized and unrealized investment losses	(17.8)	5.5
Total revenues	1,603.2	1,551.1
Current accident year losses and LAE	879.7	858.7
Prior year favorable development, excluding catastrophes	(20.0)	(10.4)
Current year catastrophe losses	107.6	93.9
Prior year favorable catastrophe development	(12.0)	(7.0)
Total losses and LAE	955.3	935.2
Amortization of deferred acquisition costs and other underwriting expenses	470.3	452.7
Underwriting income (loss)	82.9	60.7
Other segment items	(9.0)	(8.0)
Operating income before interest expense and income taxes	186.4	149.7
Interest on debt	(8.5)	(8.5)
Operating income before income taxes	177.9	141.2
Other non-operating items		(1.4)
Income before income taxes	160.1	145.3
Core Commercial [Member]
Segment Reporting Information [Line Items]
Premiums	541.0	528.9
Net investment income	48.0	40.8
Fees and other income	1.3	1.3
Total operating revenues	590.3	571.0
Current accident year losses and LAE	334.2	309.4
Prior year favorable development, excluding catastrophes	(1.3)	(9.2)
Current year catastrophe losses	54.5	24.5
Prior year favorable catastrophe development	(8.5)	(3.8)
Total losses and LAE	378.9	320.9
Amortization of deferred acquisition costs and other underwriting expenses	182.1	177.1
Underwriting income (loss)	(20.0)	30.9
Other segment items	(2.5)	(1.5)
Operating income before interest expense and income taxes	26.8	71.5
Specialty [Member]
Segment Reporting Information [Line Items]
Premiums	339.6	320.9
Net investment income	24.3	20.3
Fees and other income	1.3	1.3
Total operating revenues	365.2	342.5
Current accident year losses and LAE	173.3	156.6
Prior year favorable development, excluding catastrophes	(15.9)	(1.1)
Current year catastrophe losses	17.2	10.2
Prior year favorable catastrophe development	(2.5)	(3.2)
Total losses and LAE	172.1	162.5
Amortization of deferred acquisition costs and other underwriting expenses	126.3	119.5
Underwriting income (loss)	41.2	38.9
Other segment items	(2.2)	(1.7)
Operating income before interest expense and income taxes	64.6	58.8
Personal Lines [Member]
Segment Reporting Information [Line Items]
Premiums	627.9	598.8
Net investment income	30.5	25.6
Fees and other income	3.7	4.0
Total operating revenues	662.1	628.4
Current accident year losses and LAE	372.2	392.7
Prior year favorable development, excluding catastrophes	(2.8)	(0.1)
Current year catastrophe losses	35.9	59.2
Prior year favorable catastrophe development	(1.0)
Total losses and LAE	404.3	451.8
Amortization of deferred acquisition costs and other underwriting expenses	161.9	156.1
Underwriting income (loss)	61.7	(9.1)
Other segment items	(1.7)	(1.6)
Operating income before interest expense and income taxes	94.2	18.9
Other [Member]
Segment Reporting Information [Line Items]
Net investment income	3.3	3.0
Fees and other income	0.1	0.7
Total operating revenues	3.4	3.7
Other segment items	(2.6)	(3.2)
Operating income before interest expense and income taxes	$ 0.8	$ 0.5