Condensed Consolidated Cash Flows Statements - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net income	$ 92.2	$ 57.9
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	12.5	9.6
Bad debts	6.2	1.5
Recovery of bad debt on physical coal	0.0	(2.4)
Deferred income taxes	(0.7)	0.3
Amortization of debt issuance costs	1.3	1.2
Amortization of share-based compensation	7.4	5.9
Bargain purchase gain on acquisition	0.0	(5.4)
Changes in operating assets and liabilities, net:
Securities and other assets segregated under federal and other regulations	296.2	584.4
Securities purchased under agreements to resell	(332.8)	(662.2)
Securities borrowed	(44.2)	(948.6)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net	55.9	(238.7)
Receivables from clients, net	8.3	5.9
Notes receivable, net	(1.8)	(0.6)
Income taxes receivable	7.7	(0.6)
Financial instruments owned, at fair value	(656.7)	(368.0)
Physical commodities inventory, net	(40.9)	(115.5)
Operating right of use assets	6.0	0.0
Other assets	(7.0)	(7.8)
Accounts payable and other accrued liabilities	20.5	(4.0)
Operating lease liabilities	(6.7)	0.0
Payables to clients	848.4	(7.3)
Payables to broker-dealers, clearing organizations and counterparties	163.8	100.9
Income taxes payable	1.6	1.8
Securities sold under agreements to repurchase	756.7	964.0
Securities loaned	(0.2)	963.0
Financial instruments sold, not yet purchased, at fair value	13.5	(9.7)
Net cash provided by (used in) operating activities	1,207.2	325.6
Cash flows from investing activities:
Cash paid for acquisitions, net	(9.8)	(28.0)
Purchase of property and equipment	(5.5)	(10.3)
Net cash used in investing activities	(15.3)	(38.3)
Cash flows from financing activities:
Net change in payable to lenders under loans with maturities 90 days or less	150.4	(16.9)
Proceeds from payables to lenders under loans with maturities greater than 90 days	521.5	0.0
Repayments of payables to lenders under loans with maturities greater than 90 days	(560.0)	0.0
Proceeds from issuance of senior secured term loan	21.5	175.0
Repayments of senior secured term loan	(7.4)	(4.4)
Proceeds from issuance of senior secured notes	344.8	0.0
Repayments of note payable	(0.4)	(0.6)
Deferred payments on acquisitions	(0.6)	0.0
Repurchase of stock	(7.5)	0.0
Debt issuance costs	(9.2)	(3.3)
Exercise of stock options	5.2	0.8
Net cash (used in) provided by financing activities	458.3	150.6
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents	(2.7)	0.6
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents	1,647.5	438.5
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	2,451.3	2,190.1
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	4,098.8	2,628.6
Supplemental disclosure of cash flow information:
Cash paid for interest	78.0	110.1
Income taxes paid, net of cash refunds	25.7	17.5
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions	11.5	2.7
Assets acquired	357.8	40.9
Liabilities assumed	(359.5)	(7.7)
Total net assets acquired	$ (1.7)	$ 33.2