Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Jun. 30, 2022	Jun. 30, 2021
Cash flows from operating activities:
Net income		$ 154.8	$ 109.0
Adjustments to reconcile net income to net cash (used in)/provided by operating activities:
Depreciation and amortization		32.1	26.7
Amortization of right of use assets		12.1	6.5
Bad debt expense, net		11.4	3.7
Deferred income taxes		0.4	2.2
Amortization of debt issuance costs		3.5	2.9
Amortization of share-based compensation		12.8	10.3
Gain on sale of property and equipment		0.0	(0.5)
Bargain purchase gain on acquisition		0.0	(3.3)
Changes in operating assets and liabilities, net:
Securities and other assets segregated under federal and other regulations		(7.3)	(8.1)
Securities purchased under agreements to resell		523.7	(331.0)
Securities borrowed		1,054.4	(257.3)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net		(2,931.3)	(164.1)
Receivables from clients, net		(176.7)	(217.8)
Notes receivable, net		1.1	(4.9)
Income taxes receivable		17.4	(0.5)
Financial instruments owned, at fair value		621.1	(739.5)
Physical commodities inventory, net		(117.9)	(203.7)
Other assets		(12.4)	(13.7)
Accounts payable and other accrued liabilities		36.4	35.2
Operating lease liabilities		(13.3)	(2.4)
Payables to clients		2,311.0	1,488.5
Payables to broker-dealers, clearing organizations, and counterparties		(182.8)	(162.0)
Income taxes payable		11.8	(6.3)
Securities sold under agreements to repurchase		(1,533.2)	472.1
Securities loaned		(1,058.1)	258.4
Financial instruments sold, not yet purchased, at fair value		733.0	1,078.2
Net cash (used in)/provided by operating activities		(496.0)	1,378.6
Cash flows from investing activities:
Cash paid for acquired intangible assets and business acquisitions, net		(0.2)	(0.4)
Proceeds from sale of property, plant, and equipment		0.0	3.1
Sale of exchange memberships and common stock		0.2	0.0
Purchases of property and equipment		(37.6)	(53.5)
Net cash used in investing activities		(37.6)	(50.8)
Cash flows from financing activities:
Net change in payables to lenders under loans with maturities 90 days or less		269.6	1.6
Proceeds from payables to lenders under loans with maturities greater than 90 days		452.0	143.4
Repayments of payables to lenders under loans with maturities greater than 90 days		(462.0)	(148.4)
Repayments of senior secured term loan		(170.3)	(7.4)
Repayment of senior secured notes		0.0	(1.6)
Issuance of note payable		0.0	9.0
Deferred payments on acquisitions		(1.5)	(1.7)
Payment of contingent consideration		(2.8)	0.0
Repurchase of common stock		0.0	(3.1)
Exercise of stock options		6.2	7.6
Net cash provided by/(used in) financing activities		91.2	(0.6)
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents		(5.5)	16.7
Net (decrease)/increase in cash, segregated cash, cash equivalents, and segregated cash equivalents		(447.9)	1,343.9
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period		6,509.5	4,468.4
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period		6,061.6	5,812.3
Supplemental disclosure of cash flow information:
Cash paid for interest		85.4	58.0
Income taxes paid, net of cash refunds		25.4	44.7
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions		0.0	0.5
Acquisition of business:
Assets acquired		0.0	0.1
Liabilities assumed		0.0	(0.2)
Total net liabilities assumed		0.0	(0.1)
Cash and cash equivalents		1,363.7	1,075.6
Cash segregated under federal and other regulations	[1]	2,522.3	2,229.5
Securities segregated under federal and other regulations	[1]	0.1	75.0
Cash segregated and deposited with or pledged to exchange-clearing organizations and other futures commission merchants (“FCMs”)	[2]	1,910.0	2,245.0
Securities segregated and pledged to exchange-clearing organizations	[2]	265.5	187.2
Total cash, segregated cash, cash equivalents, and segregated cash equivalents shown in the condensed consolidated statements of cash flows		6,061.6	5,812.3
Non-cash equivalent segregated assets		21.4	21.0
Non-segregated cash and other non-cash equivalent assets included within deposits and receivables from broker-dealers, clearing organizations, and counterparties		$ 5,084.6	$ 2,185.4

[1] Represents segregated client cash held at third-party banks. Excludes segregated commodity warehouse receipts, segregated U.S. Treasury obligations with original or acquired maturities of greater than 90 days, and other assets of $21.4 million and $21.0 million as of June 30, 2022 and 2021, respectively, included within Cash, securities and other assets segregated under federal and other regulations on the Condensed Consolidated Balance Sheets.

[2] Represents segregated client cash and U.S. Treasury obligations on deposit with, or pledged to, exchange clearing organizations and other FCMs. Excludes non-segregated cash, segregated U.S. Treasury obligations pledged to exchange-clearing organizations with original or acquired maturities greater than 90 days, and other assets of $5,084.6 million and $2,185.4 million as of June 30, 2022 and 2021, respectively, included within Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net on the Condensed Consolidated Balance Sheets.