Investments - Schedule of Amortized Cost and Fair Values of Fixed Maturity Investments, by Contractual Maturity (Details) $ in Millions	Sep. 30, 2024 USD ($)
Amortized Cost
Due in one year or less	$ 304.5
Due after one year through five years	1,039.3
Due after five years through ten years	191.9
Due after ten years	20.1
Structured securities	727.8
Total	2,283.6
Fair Value
Due in one year or less	306.4
Due after one year through five years	1,067.4
Due after five years through ten years	206.8
Due after ten years	21.6
Structured securities	766.0
Total	$ 2,368.2