Segment Information - Summary of Segment Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Segment Reporting Information
Net premiums and contract charges earned	$ 289.1	$ 266.0	$ 845.2	$ 782.6
Net investment income	113.0	118.9	326.8	327.8
Net income (loss)	34.3	11.7	64.6	5.5
Assets	14,713.6		14,713.6		$ 14,049.9
Intersegment eliminations
Segment Reporting Information
Net investment income	(0.6)	(0.5)	(1.6)	(1.6)
Assets	(86.5)		(86.5)		(62.4)
Property & Casualty | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	187.3	164.3	539.7	474.1
Net investment income	8.8	12.0	30.6	26.6
Net income (loss)	10.6	(11.3)	12.6	(44.3)
Assets	1,327.0		1,327.0		1,218.1
Life & Retirement | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	39.1	38.2	114.9	113.3
Net investment income	94.6	96.9	269.3	274.1
Net income (loss)	14.8	20.9	38.8	52.3
Assets	11,893.3		11,893.3		11,365.0
Supplemental & Group Benefits | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	62.7	63.5	190.6	195.2
Net investment income	10.1	10.3	28.4	28.5
Net income (loss)	14.8	15.8	39.9	41.6
Assets	1,395.0		1,395.0		1,338.8
Corporate & Other
Segment Reporting Information
Net investment income	0.1	0.2	0.1	0.2
Net income (loss)	(5.9)	$ (13.7)	(26.7)	$ (44.1)
Assets	$ 184.8		$ 184.8		$ 190.4