Liability for Future Policy Holder Benefits - Changes in Deferred Profit Liability (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Deferred Profit Liability [Roll Forward]
Balance, beginning of period	$ 337.8	$ 253.6	$ 193.4
Annual assumption changes	4.8	15.0	(12.7)
Profits deferred	160.7	163.1	164.7
Interest accrual	17.1	13.2	10.4
Amortization	(110.7)	(111.2)	(101.6)
Effect of actual variances from expected experience and other changes	2.4	4.1	(0.6)
Balance, end of period	$ 412.1	$ 337.8	$ 253.6