Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Policyholder Account Balance [Roll Forward]
Beginning balance			$ 5,187.0
Interest credited	$ 54.8	$ 52.7	161.5	$ 152.1
Ending balance	5,128.6		5,128.6		$ 5,187.0
Indexed Universal Life Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	65.2	52.0	57.8	47.6	47.6
Premiums received	3.5	3.1	12.3	9.8
Surrenders and withdrawals	(0.9)	(0.2)	(1.7)	(0.9)
Benefit payments	0.0	0.0	0.0	0.0
Net transfers from (to) separate account	(0.2)	(0.3)	(0.3)	(0.5)
Interest credited	0.6	0.7	2.2	0.9
Other	(0.8)	(0.6)	(2.9)	(2.2)
Ending balance	$ 67.4	$ 54.7	$ 67.4	$ 54.7	57.8
Weighted average credit rate	3.70%	5.40%	4.80%	2.30%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	49.1	37.7	49.1	37.7
Experience Life Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	59.4	63.0	61.2	64.3	64.3
Premiums received	(0.2)	(0.1)	(0.5)	(0.5)
Surrenders and withdrawals	(0.8)	(1.1)	(2.9)	(2.6)
Benefit payments	(0.3)	(0.4)	(1.2)	(1.3)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.7	0.8	2.2	2.3
Other	0.0	(0.1)	0.0	(0.1)
Ending balance	$ 58.8	$ 62.1	$ 58.8	$ 62.1	61.2
Weighted average credit rate	4.90%	5.20%	5.00%	5.00%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	58.2	61.4	58.2	61.4
Fixed Annuity
Policyholder Account Balance [Roll Forward]
Beginning balance	4,509.5	4,563.5	4,556.0	4,591.1	4,591.1
Premiums received	59.8	70.0	151.6	183.9
Surrenders and withdrawals	(85.3)	(87.8)	(283.9)	(289.8)
Benefit payments	(12.6)	(21.2)	(49.5)	(58.1)
Net transfers from (to) separate account	8.8	3.0	17.0	17.5
Interest credited	42.1	40.8	125.1	120.7
Other	3.3	0.2	9.3	3.2
Ending balance	$ 4,525.6	$ 4,568.5	$ 4,525.6	$ 4,568.5	4,556.0
Weighted average credit rate	3.80%	3.60%	3.70%	3.60%
Net amount at risk	$ 29.0	$ 53.9	$ 29.0	$ 53.9
Cash surrender value	4,483.6	4,512.3	4,483.6	4,512.3
Indexed Annuity
Policyholder Account Balance [Roll Forward]
Beginning balance	426.6	481.2	449.0	510.3	510.3
Premiums received	3.7	5.3	11.4	15.8
Surrenders and withdrawals	(12.1)	(17.6)	(38.9)	(50.1)
Benefit payments	(1.7)	(0.6)	(3.3)	(2.1)
Net transfers from (to) separate account	(0.6)	(3.1)	(2.0)	(6.4)
Interest credited	2.9	3.0	8.8	3.9
Other	(5.2)	(0.2)	(11.4)	(3.4)
Ending balance	$ 413.6	$ 468.0	$ 413.6	$ 468.0	449.0
Weighted average credit rate	2.80%	2.60%	2.80%	1.10%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	406.9	456.8	406.9	456.8
Non-Life Contingent Single-Premium Immediate Annuity Insurance
Policyholder Account Balance [Roll Forward]
Beginning balance	30.4	33.5	32.6	34.4	34.4
Premiums received	0.5	0.5	1.4	2.6
Surrenders and withdrawals	0.0	0.0	(0.9)	(0.2)
Benefit payments	(1.2)	(1.2)	(3.9)	(4.3)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.2	0.3	0.7	0.8
Other	0.2	0.0	0.2	(0.2)
Ending balance	$ 30.1	$ 33.1	$ 30.1	$ 33.1	$ 32.6
Weighted average credit rate	2.70%	3.70%	3.10%	3.10%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	$ 29.6	$ 32.9	$ 29.6	$ 32.9