Allowance for Expected Credit Losses on Premiums Receivable (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss	$ 26.8	$ 28.7	$ 26.8	$ 28.7	$ 30.0	$ 29.5	$ 31.6	$ 32.5
Premiums Receivable, Gross	659.8	660.3	659.8	660.3	$ 676.9	$ 612.4	$ 635.6	$ 557.6
Premium Receivable, Credit Loss Expense (Reversal)	$ (3.2)	$ (2.9)	$ (2.7)	$ (3.8)