Liability for Future Policyholder Benefits - Changes in Deferred Profit Liability (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Deferred Profit Liability [Roll Forward]
Balance, beginning of period	$ 337.8	$ 253.6
Profits deferred	41.8	43.0
Interest accrual	4.0	3.0
Amortization	(25.9)	(27.0)
Effect of actual variances from expected experience and other changes	(0.2)	0.0
Balance, end of period	$ 357.5	$ 272.6