Segment Information - Summary of Segment Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Segment Reporting Information
Net premiums and contract charges earned	$ 260.7	$ 255.4	$ 516.6	$ 511.2
Net investment income	108.5	105.2	208.9	203.1
Net income (loss)	(12.8)	(4.2)	(6.2)	16.1
Assets	13,671.9		13,671.9		$ 13,306.1
Intersegment eliminations
Segment Reporting Information
Net investment income	(0.5)	(0.5)	(1.1)	(1.1)
Assets	(51.7)		(51.7)		(64.9)
Property & Casualty | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	157.4	149.9	309.8	300.1
Net investment income	10.6	7.7	14.6	14.9
Net income (loss)	(21.4)	(25.4)	(33.0)	(16.9)
Assets	1,077.9		1,077.9		1,083.8
Life & Retirement | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	37.4	36.3	75.1	72.1
Net investment income	89.3	88.4	177.2	172.6
Net income (loss)	17.4	23.8	31.4	39.4
Assets	11,127.1		11,127.1		10,754.5
Supplemental & Group Benefits | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	65.9	69.2	131.7	139.0
Net investment income	9.1	9.6	18.2	16.7
Net income (loss)	11.8	15.0	25.8	28.2
Assets	1,350.4		1,350.4		1,359.3
Corporate & Other
Segment Reporting Information
Net investment income	0.0	0.0	0.0	0.0
Net income (loss)	(20.6)	$ (17.6)	(30.4)	$ (34.6)
Assets	$ 168.2		$ 168.2		$ 173.4