Condensed Consolidated Cash Flows - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 19.5	$ 16.3
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Depreciation and amortization	8.4	3.9
Amortization of right of use assets	2.8
Bad debts	1.5	0.0
Deferred income taxes	0.2	(0.6)
Amortization of debt issuance costs	1.0	0.4
Amortization of share-based compensation	3.1	2.6
Changes in operating assets and liabilities, net:
Securities and other assets segregated under federal and other regulations	2.8	299.0
Securities purchased under agreements to resell	(239.4)	(178.6)
Securities borrowed	212.2	(4.5)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net	(350.8)	281.1
Receivables from clients, net	153.9	93.3
Notes receivable, net	(3.0)	(2.9)
Income taxes receivable	(4.5)	0.5
Financial instruments owned, at fair value	(124.7)	(2.2)
Physical commodities inventory, net	(158.0)	(20.4)
Other assets	(7.1)	(4.1)
Accounts payable and other accrued liabilities	(27.8)	(25.8)
Operating lease liabilities	(1.6)	(1.3)
Payables to clients	202.0	114.2
Payables to broker-dealers, clearing organizations and counterparties	(222.7)	14.4
Income taxes payable	(3.7)	0.4
Securities sold under agreements to repurchase	479.8	157.9
Securities loaned	(204.1)	(29.1)
Financial instruments sold, not yet purchased, at fair value	141.4	(48.4)
Net cash (used in) provided by operating activities	(118.8)	667.6
Cash flows from investing activities:
Cash paid for acquisitions, net	(0.4)	(5.1)
Purchase of property and equipment	(22.4)	(1.8)
Net cash used in investing activities	(22.8)	(6.9)
Cash flows from financing activities:
Net change in payables to lenders under loans with maturities 90 days or less	91.5	(106.0)
Proceeds from payables to lenders under loans with maturities greater than 90 days	67.0	180.5
Repayments of payables to lenders under loans with maturities greater than 90 days	(62.0)	(138.0)
Proceeds from issuance of senior secured term loan	0.0	21.5
Repayments of senior secured term loan	(2.4)	(2.5)
Issuance of note payable	9.0	0.0
Repayments of note payable	0.0	(0.1)
Deferred payments on acquisitions	(0.3)	0.0
Repurchase of common stock	0.0	(0.4)
Debt issuance costs	0.0	(0.2)
Exercise of stock options	1.7	1.5
Net cash provided by (used in) financing activities	104.5	(43.7)
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents	14.1	0.7
Net (decrease) increase in cash, segregated cash, cash equivalents, and segregated cash equivalents	(23.0)	617.7
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	4,468.4	2,451.3
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	4,445.4	3,069.0
Supplemental disclosure of cash flow information:
Cash paid for interest	27.5	33.0
Income taxes paid, net of cash refunds	14.4	5.2
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions	0.5	4.4
Acquisition of business:
Assets acquired	0.1	296.5
Liabilities assumed	(0.2)	(295.8)
Total net (liabilities) assets assumed	(0.1)	0.7
Total cash, segregated cash, cash equivalents, and segregated cash equivalents shown in the condensed consolidated statements of cash flows	$ 4,445.4	$ 3,069.0