Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Policyholder Account Balance [Roll Forward]
Beginning balance			$ 5,260.6
Interest credited	$ 52.7	$ 44.8	152.1	$ 125.9
Ending balance	5,204.0		5,204.0		$ 5,260.6
Indexed Universal Life
Policyholder Account Balance [Roll Forward]
Beginning balance	52.0	42.6	47.6	39.1	39.1
Premiums received	3.1	3.2	9.8	7.9
Surrenders and withdrawals	(0.2)	(0.1)	(0.9)	(0.7)
Benefit payments	0.0	0.0	0.0	0.0
Net transfers from (to) separate account	(0.3)	0.0	(0.5)	0.0
Interest credited	0.7	0.0	0.9	0.8
Other	(0.6)	(0.9)	(2.2)	(2.3)
Ending balance	$ 54.7	$ 44.8	$ 54.7	$ 44.8	47.6
Weighted-average crediting rate	5.40%	0.00%	2.30%	2.70%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	37.7	29.0	37.7	29.0
Experience Life
Policyholder Account Balance [Roll Forward]
Beginning balance	63.0	65.4	64.3	66.2	66.2
Premiums received	(0.1)	(0.2)	(0.5)	(0.2)
Surrenders and withdrawals	(1.1)	(0.9)	(2.6)	(2.4)
Benefit payments	(0.4)	(0.4)	(1.3)	(1.2)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.8	0.9	2.3	2.4
Other	(0.1)	(0.1)	(0.1)	(0.1)
Ending balance	$ 62.1	$ 64.7	$ 62.1	$ 64.7	64.3
Weighted-average crediting rate	5.20%	5.70%	5.00%	5.00%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	61.4	64.0	61.4	64.0
Fixed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance	4,563.5	4,573.7	4,591.1	4,532.7	4,532.7
Premiums received	70.0	61.3	183.9	156.0
Surrenders and withdrawals	(87.8)	(64.4)	(289.8)	(184.2)
Benefit payments	(21.2)	(17.7)	(58.1)	(50.6)
Net transfers from (to) separate account	3.0	7.8	17.5	34.6
Interest credited	40.8	39.2	120.7	116.4
Other	0.2	4.2	3.2	(0.8)
Ending balance	$ 4,568.5	$ 4,604.1	$ 4,568.5	$ 4,604.1	4,591.1
Weighted-average crediting rate	3.60%	3.50%	3.60%	3.50%
Net amount at risk	$ 53.9	$ 110.2	$ 53.9	$ 110.2
Cash surrender value	4,512.3	4,546.5	4,512.3	4,546.5
Fixed Indexed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance	481.2	518.2	510.3	522.6	522.6
Premiums received	5.3	8.0	15.8	25.4
Surrenders and withdrawals	(17.6)	(9.8)	(50.1)	(30.1)
Benefit payments	(0.6)	(1.1)	(2.1)	(2.9)
Net transfers from (to) separate account	(3.1)	(0.7)	(6.4)	(1.1)
Interest credited	3.0	0.1	3.9	3.2
Other	(0.2)	2.2	(3.4)	(0.2)
Ending balance	$ 468.0	$ 516.9	$ 468.0	$ 516.9	510.3
Weighted-average crediting rate	2.60%	0.10%	1.10%	0.80%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	456.8	501.7	456.8	501.7
SPIA (non-life contingent)
Policyholder Account Balance [Roll Forward]
Beginning balance	33.5	36.3	34.4	37.7	37.7
Premiums received	0.5	0.3	2.6	1.7
Surrenders and withdrawals	0.0	0.0	(0.2)	(0.4)
Benefit payments	(1.2)	(1.4)	(4.3)	(4.4)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.3	0.2	0.8	0.8
Other	0.0	0.0	(0.2)	0.0
Ending balance	$ 33.1	$ 35.4	$ 33.1	$ 35.4	$ 34.4
Weighted-average crediting rate	3.70%	2.30%	3.10%	3.00%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	$ 32.9	$ 35.0	$ 32.9	$ 35.0