Consolidated Statements of Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues
Premiums	$ 1,479.2	$ 1,431.1	$ 4,401.0	$ 4,222.8
Net investment income	91.8	84.2	271.9	250.5
Net realized and unrealized investment gains (losses):
Net realized losses from sales and other	(23.5)	(0.9)	(55.2)	(1.9)
Net change in fair value of equity securities	11.7	(5.2)	19.3	(13.4)
Impairments on investments:
Credit-related impairments	(0.1)		(3.3)	(6.2)
Losses on intent to sell securities	(0.5)		(2.2)	(10.3)
Impairments on investments	(0.6)		(5.5)	(16.5)
Total net realized and unrealized investment losses	(12.4)	(6.1)	(41.4)	(31.8)
Fees and other income	6.7	7.4	21.6	23.2
Total revenues	1,565.3	1,516.6	4,653.1	4,464.7
Losses and expenses
Losses and loss adjustment expenses	954.0	1,061.5	2,896.8	3,218.8
Amortization of deferred acquisition costs	307.8	296.6	910.3	878.1
Interest expense	8.5	8.5	25.6	25.6
Other operating expenses	165.3	150.9	494.1	451.3
Total losses and expenses	1,435.6	1,517.5	4,326.8	4,573.8
Income (loss) from continuing operations before income taxes	129.7	(0.9)	326.3	(109.1)
Income tax expense (benefit):
Current	36.7	(1.1)	82.4	(13.7)
Deferred	(9.1)	(8.0)	(14.1)	(21.6)
Total income tax expense (benefit)	27.6	(9.1)	68.3	(35.3)
Income (loss) from continuing operations	102.1	8.2	258.0	(73.8)
Discontinued operations (net of taxes):
Net income (loss)	$ 102.1	$ 8.6	$ 258.1	$ (72.6)
Basic:
Income (loss) from continuing operations	$ 2.84	$ 0.23	$ 7.18	$ (2.07)
Net income (loss) per share	$ 2.84	$ 0.24	$ 7.18	$ (2.03)
Weighted average shares outstanding	36.0	35.8	35.9	35.7
Diluted:
Income (loss) from continuing operations	$ 2.80	$ 0.23	$ 7.09	$ (2.07)
Net income (loss) per share	$ 2.80	$ 0.24	$ 7.10	$ (2.03)
Weighted average shares outstanding	36.5	36.1	36.4	35.7
Life [Member]
Discontinued operations (net of taxes):
Income from discontinued life businesses			$ 0.1
Basic:
Income from discontinued life businesses	$ 0	$ 0	$ 0	$ 0
Diluted:
Income from discontinued life businesses			$ 0.01
Chaucer [Member]
Discontinued operations (net of taxes):
Income from discontinued life businesses		$ 0.4	$ 0.0	$ 1.2
Basic:
Income from discontinued life businesses		$ 0.01		$ 0.04
Diluted:
Income from discontinued life businesses		$ 0.01		$ 0.04