Allowance for Credit Losses - Schedule of Premiums Receivable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Credit Loss [Abstract]
Premiums receivable	$ 191.7	$ 269.7	$ 230.7	$ 292.0
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	3.0	4.7
Current period change for estimated uncollectible premiums	3.0	5.1
Write-offs of uncollectible premiums receivable	(1.8)	(2.4)
Ending balance	$ 4.2	$ 7.4