Liability for Future Policyholder Benefits - Changes in Deferred Profit Liability (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Deferred Profit Liability [Roll Forward]
Balance, beginning of period	$ 337.8	$ 253.6
Profits deferred	121.1	123.2
Interest accrual	12.5	9.6
Amortization	(82.0)	(83.3)
Effect of actual variances from expected experience and other changes	1.2	1.8
Balance, end of period	$ 390.6	$ 304.9