Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Revenues
Premiums	$ 5,252.3	$ 4,770.2	$ 4,527.4
Net investment income	296.3	310.7	265.1
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	(26.5)	4.6	17.9
Net change in fair value of equity securities	(63.3)	119.1	13.4
Recoveries (Impairments) on Investments [Abstract]
Credit-related recoveries (impairments)	(1.9)	0.3	(9.8)
Losses on intent to sell securities	(14.8)	(1.0)	(16.5)
Recoveries (impairments) on investments	(16.7)	(0.7)	(26.3)
Total net realized and unrealized investment gains (losses)	(106.5)	123.0	5.0
Fees and other income	26.5	23.9	27.3
Total revenues	5,468.6	5,227.8	4,824.8
Losses and expenses
Losses and loss adjustment expenses	3,623.4	3,134.2	2,845.2
Amortization of deferred acquisition costs	1,093.2	982.7	951.0
Interest expense	34.1	34.0	37.1
Loss on repayment of debt			6.2
Other operating expenses	573.9	555.6	540.5
Total losses and expenses	5,324.6	4,706.5	4,380.0
Income from continuing operations before income taxes	144.0	521.3	444.8
Income tax expense (benefit):
Current	73.7	77.5	105.9
Deferred	(46.5)	23.8	(23.1)
Total income tax expense	27.2	101.3	82.8
Income from continuing operations	116.8	420.0	362.0
Discontinued operations:
Net income	$ 116.0	$ 418.7	$ 358.7
Basic:
Income from continuing operations	$ 3.28	$ 11.70	$ 9.61
Net income per share	$ 3.26	$ 11.67	$ 9.52
Weighted average shares outstanding	35.6	35.9	37.7
Diluted:
Income from continuing operations	$ 3.23	$ 11.52	$ 9.50
Net income per share	$ 3.21	$ 11.49	$ 9.42
Weighted average shares outstanding	36.1	36.4	38.1
Chaucer [Member]
Discontinued operations:
Income (loss) from discontinued businesses, net of taxes		$ 1.2	$ 0.4
Basic:
Income from Chaucer business, net of taxes		$ 0.03	$ 0.01
Diluted:
Income (loss) from discontinued businesses, net of taxes		$ 0.03	$ 0.01
Life [Member]
Discontinued operations:
Income (loss) from discontinued businesses, net of taxes	$ (0.8)	$ (2.5)	$ (3.7)
Basic:
Income from Chaucer business, net of taxes	$ (0.02)	$ (0.06)	$ (0.09)
Diluted:
Income (loss) from discontinued businesses, net of taxes	$ (0.02)	$ (0.06)	$ (0.10)