Liability for Future Policy Holder Benefits - Changes in Deferred Profit Liability (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Deferred Profit Liability [Roll Forward]
Balance, beginning of period	$ 253.6	$ 193.4	$ 112.7
Annual assumption changes	15.0	(12.7)	(4.6)
Profits deferred	163.1	164.7	176.3
Interest accrual	13.2	10.4	6.8
Amortization	(111.2)	(101.6)	(97.6)
Effect of actual variances from expected experience and other changes	4.1	(0.6)	(0.2)
Balance, end of period	$ 337.8	$ 253.6	$ 193.4