Segment Information - Summary of Segment Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Segment Reporting Information
Net premiums and contract charges earned	$ 275.2	$ 255.9
Net investment income	105.4	100.4
Net income (loss)	26.5	6.6
Assets	14,236.3		$ 14,049.9
Intersegment eliminations
Segment Reporting Information
Net investment income	(0.5)	(0.6)
Assets	(65.3)		(62.4)
Property & Casualty | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	173.2	152.4
Net investment income	12.3	4.0
Net income (loss)	10.6	(11.6)
Assets	1,229.2		1,218.1
Life & Retirement | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	37.8	37.7
Net investment income	85.8	87.9
Net income (loss)	11.7	14.0
Assets	11,568.3		11,365.0
Supplemental & Group Benefits | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	64.2	65.8
Net investment income	7.8	9.1
Net income (loss)	11.0	14.0
Assets	1,319.4		1,338.8
Corporate & Other
Segment Reporting Information
Net investment income	0.0	0.0
Net income (loss)	(6.8)	$ (9.8)
Assets	$ 184.7		$ 190.4