Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net income	$ 169.6	$ 85.1	$ 55.5
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
(Recovery) bad debt on physical coal	0.0	(2.4)	1.0
Depreciation and amortization	19.7	14.0	11.6
Amortization of operating right of use assets	9.9
Provision for bad debts, net of recoveries and impairment	18.7	2.5	3.1
Deferred income taxes	4.1	3.7	22.3
Amortization and extinguishment of debt issuance costs	6.5	1.5	1.0
Actuarial gain on pension and postretirement benefits	(0.2)	(0.3)	(0.3)
Amortization of share-based compensation expense	10.3	8.1	6.6
Gain on acquisition	(81.9)	(5.5)	0.0
Changes in operating assets and liabilities, net:
Cash, securities and other assets segregated under federal and other regulations	293.0	337.2	(626.7)
Securities purchased under agreements to resell	(271.7)	(553.7)	(464.9)
Securities borrowed	(16.8)	(1,197.7)	(138.8)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	(326.0)	(241.7)	(292.9)
Receivable from clients, net	0.9	(134.3)	(24.3)
Notes receivable, net	1.2	0.9	6.8
Income taxes receivable	(11.1)	(4.2)	(1.3)
Financial instruments owned, at fair value	(552.5)	(113.3)	(308.7)
Physical commodities inventory	(51.8)	3.0	(98.7)
Other assets	(3.7)	(8.3)	(3.3)
Accounts payable and other accrued liabilities	42.7	6.7	18.6
Operating lease liabilities	(9.5)
Payable to clients	2,093.7	(46.8)	520.0
Payable to broker-dealers, clearing organizations and counterparties	270.8	176.4	(27.8)
Income taxes payable	(0.3)	1.8	3.2
Securities sold under agreements to repurchase	381.8	837.0	543.7
Securities loaned	(18.0)	1,182.0	166.8
Financial instruments sold, not yet purchased, at fair value	(28.8)	(156.1)	153.9
Net cash provided by (used in) operating activities	1,950.6	195.6	(473.6)
Cash flows from investing activities:
Cash paid for acquisitions	(225.0)	(28.9)	(3.7)
Sale of clearing organization common stock	0.0	0.0	0.8
Purchase of property and equipment	(16.6)	(11.9)	(12.5)
Net cash used in investing activities	(241.6)	(40.8)	(15.4)
Cash flows from financing activities:
Net change in lenders under loans with maturities 90 days or less	99.7	(162.6)	125.8
Proceeds from lenders under loans with maturities greater than 90 days	608.5	357.2	0.0
Repayments of lenders under loans with maturities greater than 90 days	(642.0)	(346.7)	0.0
Proceeds from issuance of senior secured term loan	21.5	175.0	0.0
Repayments of senior secured term loan	(9.8)	(6.6)	0.0
Proceeds from issuance of senior secured notes	344.8	0.0	0.0
Repayment of senior secured notes	(92.1)	0.0	0.0
Repayments of note payable	(0.4)	(0.8)	(0.8)
Deferred payments on acquisitions	(0.9)	0.0	(5.5)
Share repurchase	(7.5)	(3.8)	0.0
Debt issuance costs	(15.0)	(3.3)	(0.4)
Exercise of stock options	5.5	1.2	2.6
Withholding taxes on stock option exercises	0.0	0.0	(0.8)
Net cash provided by financing activities	312.3	9.6	120.9
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents	(4.2)	(0.7)	(4.1)
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents	2,017.1	163.7	(372.2)
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	2,451.3	2,287.6	2,659.8
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	4,468.4	2,451.3	2,287.6
Supplemental disclosure of cash flow information:
Cash paid for interest	90.4	153.2	78.9
Income taxes paid, net of cash refunds	44.0	24.6	22.2
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions	11.8	10.8	3.9
Additional consideration payable related to acquisitions	21.6	1.8	0.0
Acquisition of businesses:
Assets acquired	1,169.2	47.1	1.7
Liabilities acquired	(359.5)	(8.9)	(1.9)
Total net assets acquired	809.7	38.2	(0.2)
Total cash, segregated cash, cash equivalents, and segregated cash equivalents shown in the consolidated statements of cash flows	$ 4,468.4	$ 2,287.6	$ 2,287.6