Allowance for Expected Credit Losses on Premiums Receivable (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss	$ 29.7	$ 31.4	$ 29.7	$ 31.4	$ 28.7	$ 32.5	$ 34.3	$ 34.2
Premiums Receivable, Gross	626.6	561.1	626.6	561.1	$ 660.3	$ 557.6	$ 593.4	$ 530.7
Premium Receivable, Credit Loss Expense (Reversal)	$ 1.0	$ (2.9)	$ (2.8)	$ (2.8)