Long-Duration Insurance Contracts - Separate Account Liability Rollforward (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Separate Account, Liability [Roll Forward]
Balance, beginning of year	$ 3,294.1
Balance, end of period	3,516.9	$ 3,294.1
Variable Account Annuities
Separate Account, Liability [Roll Forward]
Balance, beginning of year	3,294.1	2,792.3
Deposits	59.3	234.2
Withdrawals	(69.7)	(213.4)
Net transfers	(1.1)	(15.5)
Fees and charges	(12.3)	(37.6)
Market appreciation (depreciation)	246.6	541.5
Other	0.0	(7.4)
Balance, end of period	$ 3,516.9	$ 3,294.1