Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Policyholder Account Balance [Roll Forward]
Beginning balance			$ 5,187.0
Interest credited	$ 53.8	$ 50.7	106.7	$ 99.4
Ending balance	5,119.0		5,119.0		$ 5,187.0
Indexed Universal Life Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	61.0	49.9	57.8	47.6	47.6
Premiums received	4.9	3.1	8.8	6.7
Surrenders and withdrawals	(0.3)	(0.3)	(0.8)	(0.7)
Benefit payments	0.0	0.0	0.0	0.0
Net transfers from (to) separate account	0.0	(0.2)	(0.1)	(0.2)
Interest credited	0.8	0.2	1.6	0.2
Other	(1.2)	(0.7)	(2.1)	(1.6)
Ending balance	$ 65.2	$ 52.0	$ 65.2	$ 52.0	57.8
Weighted average credit rate	5.20%	1.60%	5.40%	0.80%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	47.1	32.9	47.1	32.9
Experience Life Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	60.1	63.5	61.2	64.3	64.3
Premiums received	(0.2)	(0.2)	(0.3)	(0.4)
Surrenders and withdrawals	(0.8)	(0.6)	(2.1)	(1.5)
Benefit payments	(0.4)	(0.4)	(0.9)	(0.9)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.8	0.7	1.5	1.5
Other	(0.1)	0.0	0.0	0.0
Ending balance	$ 59.4	$ 63.0	$ 59.4	$ 63.0	61.2
Weighted average credit rate	5.50%	4.50%	5.00%	4.80%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	58.8	62.8	58.8	62.8
Fixed Annuity
Policyholder Account Balance [Roll Forward]
Beginning balance	4,519.9	4,577.4	4,556.0	4,591.1	4,591.1
Premiums received	42.8	59.0	91.8	113.9
Surrenders and withdrawals	(86.4)	(103.9)	(198.6)	(202.0)
Benefit payments	(16.3)	(17.8)	(36.9)	(36.9)
Net transfers from (to) separate account	6.3	7.9	8.2	14.5
Interest credited	41.3	39.8	83.0	79.9
Other	1.9	1.1	6.0	3.0
Ending balance	$ 4,509.5	$ 4,563.5	$ 4,509.5	$ 4,563.5	4,556.0
Weighted average credit rate	3.70%	3.50%	3.70%	3.60%
Net amount at risk	$ 30.9	$ 16.2	$ 30.9	$ 16.2
Cash surrender value	4,466.3	4,650.8	4,466.3	4,650.8
Indexed Annuity
Policyholder Account Balance [Roll Forward]
Beginning balance	437.4	498.4	449.0	510.3	510.3
Premiums received	4.1	5.4	7.7	10.5
Surrenders and withdrawals	(13.5)	(18.6)	(26.8)	(32.5)
Benefit payments	(0.8)	(0.9)	(1.6)	(1.5)
Net transfers from (to) separate account	(0.6)	(2.4)	(1.4)	(3.3)
Interest credited	3.2	0.9	5.9	0.9
Other	(3.2)	(1.6)	(6.2)	(3.2)
Ending balance	$ 426.6	$ 481.2	$ 426.6	$ 481.2	449.0
Weighted average credit rate	3.00%	0.70%	2.70%	0.40%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	428.4	486.7	428.4	486.7
Non-Life Contingent Single-Premium Immediate Annuity Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	31.6	33.8	32.6	34.4	34.4
Premiums received	0.1	1.0	0.9	2.1
Surrenders and withdrawals	(0.3)	(0.1)	(0.9)	(0.2)
Benefit payments	(1.3)	(1.4)	(2.7)	(3.1)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.3	0.2	0.5	0.5
Other	0.0	0.0	0.0	(0.2)
Ending balance	$ 30.4	$ 33.5	$ 30.4	$ 33.5	$ 32.6
Weighted average credit rate	3.90%	2.40%	3.10%	3.00%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	$ 31.4	$ 33.7	$ 31.4	$ 33.7