Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues
Premiums	$ 5,663.1	$ 5,252.3	$ 4,770.2
Net investment income	332.1	296.3	310.7
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	(8.9)	(26.5)	4.6
Net change in fair value of equity securities	(5.6)	(63.3)	119.1
Recoveries (Impairments) on Investments [Abstract]
Credit-related recoveries (impairments)	(7.7)	(1.9)	0.3
Losses on intent to sell securities	(10.3)	(14.8)	(1.0)
Recoveries (impairments) on investments	(18.0)	(16.7)	(0.7)
Total net realized and unrealized investment gains (losses)	(32.5)	(106.5)	123.0
Fees and other income	30.8	26.5	23.9
Total revenues	5,993.5	5,468.6	5,227.8
Losses and expenses
Losses and loss adjustment expenses	4,134.6	3,623.4	3,134.2
Amortization of deferred acquisition costs	1,176.0	1,093.2	982.7
Interest expense	34.1	34.1	34.0
Other operating expenses	607.7	573.9	555.6
Total losses and expenses	5,952.4	5,324.6	4,706.5
Income from continuing operations before income taxes	41.1	144.0	521.3
Income tax expense (benefit):
Current	32.6	73.7	77.5
Deferred	(25.0)	(46.5)	23.8
Total income tax expense	7.6	27.2	101.3
Income from continuing operations	33.5	116.8	420.0
Discontinued operations:
Net income	$ 35.3	$ 116.0	$ 422.8
Basic:
Income from continuing operations	$ 0.94	$ 3.28	$ 11.70
Net income per share	$ 0.99	$ 3.26	$ 11.78
Weighted average shares outstanding	35.7	35.6	35.9
Diluted:
Income from continuing operations	$ 0.93	$ 3.23	$ 11.52
Net income per share	$ 0.98	$ 3.21	$ 11.6
Weighted average shares outstanding	36.1	36.1	36.4
Chaucer [Member]
Discontinued operations:
Income (loss) from discontinued businesses, net of taxes	$ 1.2		$ 1.2
Basic:
Income (loss) from discontinued businesses, net of taxes	$ 0.03		$ 0.03
Diluted:
Income (loss) from discontinued businesses, net of taxes	$ 0.03		$ 0.03
Life [Member]
Discontinued operations:
Income (loss) from discontinued businesses, net of taxes	$ 0.6	$ (0.8)	$ 1.6
Basic:
Income (loss) from discontinued businesses, net of taxes	$ 0.02	$ (0.02)	$ 0.05
Diluted:
Income (loss) from discontinued businesses, net of taxes	$ 0.02	$ (0.02)	$ 0.05