Segment Information - Summarized Financial Information by Segment (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2022		Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount										$ 1,057.1	$ 1,027.7	[1]	$ 888.8	[1]
Net investment income										444.8	400.9	[1]	422.5	[1]
Net income (loss)	$ (16.7)		$ 20.4	$ (4.2)	$ 20.3	$ 46.9	$ 20.2	$ 54.2	$ 49.1	45.0	19.8	[1],[2]	170.4	[1],[2]
Assets	13,306.1	[3]				14,460.2				14,049.9	13,306.1	[3]	14,460.2
Intersegment eliminations
Summarized financial information for these segments
Net investment income										(2.2)	(2.1)		(2.3)
Assets	(64.8)					(64.8)				(62.4)	(64.8)		(64.8)
Property & Casualty | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount										645.6	608.2		617.4
Net investment income										37.9	31.4		61.1
Net income (loss)										(35.5)	(44.4)		57.0
Assets	1,083.8					1,243.4				1,218.1	1,083.8		1,243.4
Life & Retirement | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount										151.7	144.0		143.4
Net investment income										369.9	338.3		338.6
Net income (loss)										71.5	63.8		89.1
Assets	10,754.4					12,144.2				11,365.0	10,754.4		12,144.2
Supplemental & Group Benefits | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount										259.8	275.5		128.0
Net investment income										38.9	33.3		25.2
Net income (loss)										54.9	65.9		52.9
Assets	1,359.3					855.6				1,338.8	1,359.3		855.6
Corporate & Other
Summarized financial information for these segments
Net investment income										0.3	0.0		(0.1)
Net income (loss)										(45.9)	(65.5)		(28.6)
Assets	$ 173.4					$ 281.8				$ 190.4	$ 173.4		$ 281.8

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[2] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[3] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements