Long-Duration Insurance Contracts - Policyholder Account Balances (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Policyholder Account Balance [Roll Forward]
Beginning balance	[1]	$ 5,260.6
Interest credited		205.7	$ 173.4	[2]	$ 160.0	[2]
Ending balance		5,187.0	5,260.6	[1]
Indexed Universal Life
Policyholder Account Balance [Roll Forward]
Beginning balance		47.6	39.1
Premiums received		13.6	11.8
Surrenders and withdrawals		(1.1)	(1.0)
Benefit payments		0.0	0.0
Net transfers from (to) separate account		(0.6)	0.0
Interest credited		1.5	0.8
Other		(3.2)	(3.1)
Ending balance		$ 57.8	$ 47.6		39.1
Weighted-average crediting rate		2.80%	1.90%
Net amount at risk		$ 0.0	$ 0.0
Cash surrender value		40.5	30.9
Experience life
Policyholder Account Balance [Roll Forward]
Beginning balance		64.3	66.2
Premiums received		(0.8)	(0.3)
Surrenders and withdrawals		(3.7)	(3.0)
Benefit payments		(1.7)	(1.8)
Net transfers from (to) separate account		0.0	0.0
Interest credited		3.1	3.2
Other		0.0	0.0
Ending balance		$ 61.2	$ 64.3		66.2
Weighted-average crediting rate		5.00%	5.00%
Net amount at risk		$ 0.0	$ 0.0
Cash surrender value		60.5	63.6
Fixed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance		4,591.1	4,532.7
Premiums received		236.3	209.4
Surrenders and withdrawals		(391.8)	(281.5)
Benefit payments		(75.1)	(65.9)
Net transfers from (to) separate account		23.7	40.5
Interest credited		162.0	156.0
Other		9.8	(0.1)
Ending balance		$ 4,556.0	$ 4,591.1		4,532.7
Weighted-average crediting rate		3.60%	3.50%
Net amount at risk		$ 35.9	$ 83.9
Cash surrender value		4,507.5	4,535.2
Fixed Indexed Account Annuities
Policyholder Account Balance [Roll Forward]
Beginning balance		510.3	522.6
Premiums received		20.1	32.4
Surrenders and withdrawals		(67.0)	(40.4)
Benefit payments		(3.1)	(4.1)
Net transfers from (to) separate account		(8.2)	(2.5)
Interest credited		5.3	3.3
Other		(8.4)	(1.0)
Ending balance		$ 449.0	$ 510.3		522.6
Weighted-average crediting rate		1.10%	0.60%
Net amount at risk		$ 0.0	$ 0.0
Cash surrender value		439.9	496.3
SPIA (non-life contingent)
Policyholder Account Balance [Roll Forward]
Beginning balance		34.4	37.7
Premiums received		3.4	2.4
Surrenders and withdrawals		(0.4)	(0.5)
Benefit payments		(5.9)	(6.3)
Net transfers from (to) separate account		0.0	0.0
Interest credited		1.0	1.1
Other		0.1	0.0
Ending balance		$ 32.6	$ 34.4		$ 37.7
Weighted-average crediting rate		3.10%	3.00%
Net amount at risk		$ 0.0	$ 0.0
Cash surrender value		$ 32.3	$ 34.1

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[2] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements