Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Policyholder Account Balance [Roll Forward]
Beginning balance			$ 5,100.3
Interest credited	$ 55.7	$ 54.8	161.2	$ 161.5
Ending balance	5,092.0		5,092.0		$ 5,100.3
Indexed Universal Life
Policyholder Account Balance [Roll Forward]
Beginning balance	82.3	65.2	72.9	57.8	57.8
Premiums received	5.8	3.5	16.7	12.3
Surrenders and withdrawals	(0.7)	(0.9)	(1.5)	(1.7)
Benefit payments	0.0	0.0	0.0	0.0
Net transfers from (to) separate account	(0.1)	(0.2)	(0.3)	(0.3)
Interest credited	1.2	0.6	3.0	2.2
Other	(1.3)	(0.8)	(3.6)	(2.9)
Ending balance	$ 87.2	$ 67.4	$ 87.2	$ 67.4	72.9
Weighted-average crediting rate	5.80%	3.70%	5.10%	4.80%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	66.6	49.1	66.6	49.1
Experience life
Policyholder Account Balance [Roll Forward]
Beginning balance	56.5	59.4	58.0	61.2	61.2
Premiums received	(0.2)	(0.2)	(0.6)	(0.5)
Surrenders and withdrawals	(0.9)	(0.8)	(2.7)	(2.9)
Benefit payments	(0.3)	(0.3)	(1.0)	(1.2)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.7	0.7	2.1	2.2
Other	(0.1)	0.0	(0.1)	0.0
Ending balance	$ 55.7	$ 58.8	$ 55.7	$ 58.8	58.0
Weighted-average crediting rate	5.10%	4.90%	5.00%	5.00%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	55.1	58.2	55.1	58.2
Fixed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance	4,495.1	4,509.5	4,508.4	4,556.0	4,556.0
Premiums received	61.3	59.8	151.3	151.6
Surrenders and withdrawals	(77.0)	(85.3)	(247.0)	(283.9)
Benefit payments	(14.9)	(12.6)	(58.1)	(49.5)
Net transfers from (to) separate account	3.6	8.8	23.8	17.0
Interest credited	42.6	42.1	126.2	125.1
Other	3.9	3.3	10.0	9.3
Ending balance	$ 4,514.6	$ 4,525.6	$ 4,514.6	$ 4,525.6	4,508.4
Weighted-average crediting rate	3.90%	3.80%	3.80%	3.70%
Net amount at risk	$ 23.9	$ 29.0	$ 23.9	$ 29.0
Cash surrender value	4,470.7	4,483.6	4,470.7	4,483.6
Fixed Indexed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance	388.6	426.6	409.5	449.0	449.0
Premiums received	3.1	3.7	10.7	11.4
Surrenders and withdrawals	(9.3)	(12.1)	(33.7)	(38.9)
Benefit payments	(0.6)	(1.7)	(3.0)	(3.3)
Net transfers from (to) separate account	(0.2)	(0.6)	(0.8)	(2.0)
Interest credited	4.0	2.9	10.2	8.8
Other	(3.3)	(5.2)	(10.6)	(11.4)
Ending balance	$ 382.3	$ 413.6	$ 382.3	$ 413.6	409.5
Weighted-average crediting rate	4.20%	2.80%	3.50%	2.80%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	377.2	406.9	377.2	406.9
SPIA (non-life contingent)
Policyholder Account Balance [Roll Forward]
Beginning balance	27.9	30.4	28.6	32.6	32.6
Premiums received	0.3	0.5	1.7	1.4
Surrenders and withdrawals	(0.1)	0.0	(0.3)	(0.9)
Benefit payments	(0.9)	(1.2)	(3.2)	(3.9)
Net transfers from (to) separate account	0.0	0.0	0.0	0.0
Interest credited	0.2	0.2	0.7	0.7
Other	0.0	0.2	(0.1)	0.2
Ending balance	$ 27.4	$ 30.1	$ 27.4	$ 30.1	$ 28.6
Weighted-average crediting rate	2.90%	2.70%	3.60%	3.10%
Net amount at risk	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Cash surrender value	$ 27.0	$ 29.6	$ 27.0	$ 29.6