Consolidated Statements of Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenues
Premiums	$ 1,448.6	$ 1,380.0
Net investment income	89.7	78.7
Net realized and unrealized investment gains (losses):
Net realized losses from sales and other	(1.3)	(1.1)
Net change in fair value of equity securities	6.5	(7.1)
Recoveries (Impairments) on Investments [Abstract]
Credit-related recoveries (impairments)	0.3	(4.5)
Losses on intent to sell securities		(10.3)
Recoveries (impairments) on investments	0.3	(14.8)
Total net realized and unrealized investment gains (losses)	5.5	(23.0)
Fees and other income	7.3	8.0
Total revenues	1,551.1	1,443.7
Losses and expenses
Losses and loss adjustment expenses	935.2	1,017.4
Amortization of deferred acquisition costs	299.0	288.8
Interest expense	8.5	8.5
Other operating expenses	163.1	146.5
Total losses and expenses	1,405.8	1,461.2
Income (loss) before income taxes	145.3	(17.5)
Income tax expense (benefit):
Current	28.7	2.2
Deferred	1.1	(7.7)
Total income tax expense (benefit)	29.8	(5.5)
Net income (loss)	$ 115.5	$ (12.0)
Basic:
Net income (loss) per share	$ 3.22	$ (0.34)
Weighted average shares outstanding	35.8	35.6
Diluted:
Net income (loss) per share	$ 3.18	$ (0.34)
Weighted average shares outstanding	36.3	35.6