Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 1,346.6	$ 1,244.6
Net realized and unrealized investment gains (losses)	(15.9)	37.5
Total revenues	1,330.7	1,282.1
Net investment income	76.9	76.8
Operating income (loss) before interest expense and income taxes	154.4	85.1
Interest on debt	(8.5)	(8.5)
Operating income before income taxes	145.9	76.6
Income from continuing operations before income taxes	130.0	114.1
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	511.1	472.9
Underwriting (loss) income	32.2	(51.3)
Net investment income	35.4	36.9
Other income (expense)	(0.1)	(0.4)
Operating income (loss) before interest expense and income taxes	67.5	(14.8)
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	301.3	274.2
Underwriting (loss) income	33.9	2.0
Net investment income	16.2	14.9
Other income (expense)	(0.1)	0.1
Operating income (loss) before interest expense and income taxes	50.0	17.0
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	530.7	493.3
Underwriting (loss) income	12.3	58.9
Net investment income	22.6	22.1
Other income (expense)	1.4	0.8
Operating income (loss) before interest expense and income taxes	36.3	81.8
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.5	4.2
Underwriting (loss) income		(0.3)
Net investment income	2.7	2.9
Other income (expense)	(2.1)	(1.5)
Operating income (loss) before interest expense and income taxes	$ 0.6	$ 1.1