Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Policyholder Account Balance [Roll Forward]
Beginning balance	$ 5,187.0
Interest credited	52.9	$ 48.7
Ending balance	5,137.6		$ 5,187.0
Indexed Universal Life Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	57.8	47.6	47.6
Premiums received	3.9	3.6
Surrenders and withdrawals	(0.5)	(0.4)
Benefit payments	0.0	0.0
Net transfers from (to) separate account	(0.1)	0.0
Interest credited	0.8	0.0
Other	(0.9)	(0.9)
Ending balance	$ 61.0	$ 49.9	57.8
Weighted average credit rate	5.60%	0.10%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	43.3	32.9
Experience Life Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	61.2	64.3	64.3
Premiums received	(0.1)	(0.2)
Surrenders and withdrawals	(1.3)	(0.9)
Benefit payments	(0.5)	(0.5)
Net transfers from (to) separate account	0.0	0.0
Interest credited	0.7	0.8
Other	0.1	0.0
Ending balance	$ 60.1	$ 63.5	61.2
Weighted average credit rate	4.90%	5.00%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	59.5	62.8
Fixed Annuity
Policyholder Account Balance [Roll Forward]
Beginning balance	4,556.0	4,591.1	4,591.1
Premiums received	49.0	54.9
Surrenders and withdrawals	(112.2)	(98.1)
Benefit payments	(20.6)	(19.1)
Net transfers from (to) separate account	1.9	6.6
Interest credited	41.7	40.1
Other	4.1	1.9
Ending balance	$ 4,519.9	$ 4,577.4	4,556.0
Weighted average credit rate	3.70%	3.60%
Net amount at risk	$ 32.0	$ 39.5
Cash surrender value	4,468.2	4,540.9
Indexed Annuity
Policyholder Account Balance [Roll Forward]
Beginning balance	449.0	510.3	510.3
Premiums received	3.6	5.1
Surrenders and withdrawals	(13.3)	(13.9)
Benefit payments	(0.8)	(0.6)
Net transfers from (to) separate account	(0.8)	(0.9)
Interest credited	2.7	0.0
Other	(3.0)	(1.6)
Ending balance	$ 437.4	$ 498.4	449.0
Weighted average credit rate	2.50%	0.00%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	428.4	486.7
Non-Life Contingent Single-Premium Immediate Annuity Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	32.6	34.4	34.4
Premiums received	0.8	1.1
Surrenders and withdrawals	(0.6)	(0.1)
Benefit payments	(1.4)	(1.7)
Net transfers from (to) separate account	0.0	0.0
Interest credited	0.2	0.3
Other	0.0	(0.2)
Ending balance	$ 31.6	$ 33.8	$ 32.6
Weighted average credit rate	3.10%	3.00%
Net amount at risk	$ 0.0	$ 0.0
Cash surrender value	$ 31.4	$ 33.7