Segment Information - Summary of Segment Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Segment Reporting Information
Net premiums and contract charges earned	$ 280.9	$ 260.7	$ 556.1	$ 516.6
Net investment income	108.4	108.5	213.8	208.9
Net income (loss)	3.8	(12.8)	30.3	(6.2)
Assets	14,282.1		14,282.1		$ 14,049.9
Intersegment eliminations
Segment Reporting Information
Net investment income	(0.5)	(0.5)	(1.0)	(1.1)
Assets	(58.9)		(58.9)		(62.4)
Property & Casualty | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	179.2	157.4	352.4	309.8
Net investment income	9.5	10.6	21.8	14.6
Net income (loss)	(8.6)	(21.4)	2.0	(33.0)
Assets	1,255.9		1,255.9		1,218.1
Life & Retirement | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	38.0	37.4	75.8	75.1
Net investment income	88.9	89.3	174.7	177.2
Net income (loss)	12.3	17.4	24.0	31.4
Assets	11,583.2		11,583.2		11,365.0
Supplemental & Group Benefits | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	63.7	65.9	127.9	131.7
Net investment income	10.5	9.1	18.3	18.2
Net income (loss)	14.1	11.8	25.1	25.8
Assets	1,323.6		1,323.6		1,338.8
Corporate & Other
Segment Reporting Information
Net investment income	0.0	0.0	0.0	0.0
Net income (loss)	(14.0)	$ (20.6)	(20.8)	$ (30.4)
Assets	$ 178.3		$ 178.3		$ 190.4