Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Policyholder Account Balance [Roll Forward]
Beginning balance	$ 5,187.0
Interest credited	215.9	$ 205.7	$ 173.4	[1]
Ending balance	5,100.3	5,187.0
Indexed Universal Life
Policyholder Account Balance [Roll Forward]
Beginning balance	57.8	47.6
Premiums received	17.8	13.6
Surrenders and withdrawals	(2.2)	(1.1)
Benefit payments	0.0	0.0
Net transfers from (to) separate account	(0.2)	(0.6)
Interest credited	3.9	1.5
Other	(4.2)	(3.2)
Ending balance	$ 72.9	$ 57.8	47.6
Weighted-average crediting rate	6.10%	2.80%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	54.5	40.5
Experience Life
Policyholder Account Balance [Roll Forward]
Beginning balance	61.2	64.3
Premiums received	(0.8)	(0.8)
Surrenders and withdrawals	(3.7)	(3.7)
Benefit payments	(1.6)	(1.7)
Net transfers from (to) separate account	0.0	0.0
Interest credited	2.9	3.1
Other	0.0	0.0
Ending balance	$ 58.0	$ 61.2	64.3
Weighted-average crediting rate	5.00%	5.00%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	57.4	60.5
Fixed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance	4,556.0	4,591.1
Premiums received	203.5	236.3
Surrenders and withdrawals	(388.6)	(391.8)
Benefit payments	(69.2)	(75.1)
Net transfers from (to) separate account	23.3	23.7
Interest credited	167.7	162.0
Other	15.7	9.8
Ending balance	$ 4,508.4	$ 4,556.0	4,591.1
Weighted-average crediting rate	3.80%	3.60%
Net amount at risk	$ 29.4	$ 35.9
Cash surrender value	4,452.6	4,507.5
Fixed Indexed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance	449.0	510.3
Premiums received	15.9	20.1
Surrenders and withdrawals	(51.2)	(67.0)
Benefit payments	(4.2)	(3.1)
Net transfers from (to) separate account	(2.2)	(8.2)
Interest credited	16.1	5.3
Other	(13.9)	(8.4)
Ending balance	$ 409.5	$ 449.0	510.3
Weighted-average crediting rate	3.80%	1.10%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	402.0	439.9
SPIA (non-life contingent)
Policyholder Account Balance [Roll Forward]
Beginning balance	32.6	34.4
Premiums received	1.9	3.4
Surrenders and withdrawals	(1.3)	(0.4)
Benefit payments	(5.5)	(5.9)
Net transfers from (to) separate account	0.0	0.0
Interest credited	0.9	1.0
Other	0.0	0.1
Ending balance	$ 28.6	$ 32.6	$ 34.4
Weighted-average crediting rate	3.10%	3.10%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	$ 28.3	$ 32.3

[1] Recast for the adoption of ASU 2018-12.