Segment Information - Additional Significant Financial Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Segment Reporting Information [Line Items]
DAC amortization expense	$ 101.2	$ 88.2	[1]	$ 90.6	[1]
Income tax expense (benefit)	8.3	(3.3)	[1]	39.7	[1]
Property & Casualty | Operating Segments
Segment Reporting Information [Line Items]
DAC amortization expense	71.3	64.3		67.7
Income tax expense (benefit)	(9.8)	(13.8)		13.2
Life & Retirement | Operating Segments
Segment Reporting Information [Line Items]
DAC amortization expense	28.0	23.0		22.0
Income tax expense (benefit)	15.1	10.6		19.3
Supplemental & Group Benefits | Operating Segments
Segment Reporting Information [Line Items]
DAC amortization expense	1.9	0.9		0.9
Income tax expense (benefit)	14.8	18.1		14.5
Corporate & Other | Corporate and Other
Segment Reporting Information [Line Items]
Income tax expense (benefit)	$ (11.8)	$ (18.2)		$ (7.3)

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements