Allowance for Credit Losses - Reinsurance Recoverables (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Credit Loss [Abstract]
Reinsurance Recoverables, Net of Allowance for Estimated Uncollectible Reinsurance	$ 2,933.1	$ 2,959.3	$ 3,016.9	$ 3,029.1
Allowance for Estimated Uncollectible Reinsurance	$ 0.0	$ 0.0	$ 4.7	$ 4.7