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Acquisitions - R.J. O’Brien - Schedule of Unaudited Pro Forma Financial Information (Details) - RJO - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Business Combination, Pro Forma Information [Line Items]
Total revenues	$ 133,102.0	$ 100,784.7
Operating revenues	4,850.7	4,333.1
Net income	$ 329.2	$ 345.6
Basic earnings per share (in dollars per share)	$ 6.37	$ 6.84
Diluted earnings per share (in dollars per share)	$ 6.05	$ 6.62

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