Segment Information - Summary of Segment Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Segment Reporting Information
Net premiums and contract charges earned	$ 257.8	$ 225.4	$ 769.5	$ 678.8
Net investment income	97.6	103.7	300.7	308.4
Net income (loss)	13.9	16.3	15.9	102.3
Assets	13,314.1		13,314.1		$ 14,383.9
Intersegment eliminations
Segment Reporting Information
Net investment income	(0.5)	(0.6)	(1.6)	(1.8)
Assets	(67.0)		(67.0)		(64.8)
Property & Casualty | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	152.4	153.3	452.5	464.1
Net investment income	8.0	11.3	22.9	43.8
Net income (loss)	(2.5)	(4.7)	(19.4)	42.5
Assets	1,052.1		1,052.1		1,243.4
Life & Retirement | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	37.1	40.4	109.7	118.3
Net investment income	81.4	85.8	254.0	247.4
Net income (loss)	12.7	19.1	41.8	47.0
Assets	10,719.4		10,719.4		12,064.7
Supplemental & Group Benefits | Operating segments
Segment Reporting Information
Net premiums and contract charges earned	68.3	31.7	207.3	96.4
Net investment income	8.7	7.2	25.4	19.0
Net income (loss)	19.2	11.5	43.6	34.8
Assets	1,431.7		1,431.7		858.8
Corporate & Other
Segment Reporting Information
Net investment income	0.0	0.0	0.0	0.0
Net income (loss)	(15.5)	$ (9.6)	(50.1)	$ (22.0)
Assets	$ 177.9		$ 177.9		$ 281.8