Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Operating revenues	$ 1,466.7	$ 1,346.6
Net realized and unrealized investment gains (losses)	(23.0)	(15.9)
Total revenues	1,443.7	1,330.7
Net investment income	78.7	76.9
Operating income (loss) before interest expense and income taxes	13.2	154.4
Interest on debt	(8.5)	(8.5)
Operating income before income taxes	4.7	145.9
Other non-operating	0.8
Income (loss) from continuing operations before income taxes	(17.5)	130.0
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	544.7	511.1
Underwriting (loss) income	(24.8)	(32.2)
Net investment income	36.1	35.4
Other income (expense)	(0.1)	(0.1)
Operating income (loss) before interest expense and income taxes	11.2	67.5
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	330.5	301.3
Underwriting (loss) income	30.7	33.9
Net investment income	17.0	16.2
Other income (expense)	0.6	(0.1)
Operating income (loss) before interest expense and income taxes	48.3	50.0
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	587.8	530.7
Underwriting (loss) income	(72.2)	12.3
Net investment income	22.6	22.6
Other income (expense)	3.0	1.4
Operating income (loss) before interest expense and income taxes	(46.6)	36.3
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.7	3.5
Net investment income	3.0	2.7
Other income (expense)	(2.7)	(2.1)
Operating income (loss) before interest expense and income taxes	$ 0.3	$ 0.6