Condensed Consolidated Cash Flows Statements - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net income	$ 55.6	$ 41.6
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization	8.1	6.1
Bad debts	4.4	1.0
Recovery of bad debt on physical coal	0.0	(2.4)
Deferred income taxes	(1.0)	(0.4)
Amortization of debt issuance costs	0.7	0.8
Amortization of share-based compensation	5.0	3.8
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	297.9	407.5
Securities purchased under agreements to resell	164.5	(371.6)
Securities borrowed	359.4	(1,064.3)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net	(52.2)	5.8
Receivables from clients, net	(63.2)	(17.2)
Notes receivable, net	(1.0)	1.2
Income taxes receivable	8.7	0.1
Financial instruments owned, at fair value	(350.5)	(534.8)
Physical commodities inventory, net	(25.9)	(57.3)
Operating right of use assets	3.9	0.0
Other assets	0.0	(9.6)
Accounts payable and other accrued liabilities	30.2	(19.7)
Operating lease liabilities	(4.0)	0.0
Payables to clients	942.0	(642.6)
Payables to broker-dealers, clearing organizations and counterparties	163.0	485.4
Income taxes payable	0.1	2.7
Securities sold under agreements to repurchase	26.6	448.0
Securities loaned	(391.1)	1,098.6
Financial instruments sold, not yet purchased, at fair value	(11.2)	85.8
Net cash provided by (used in) operating activities	1,170.0	(136.9)
Cash flows from investing activities:
Cash paid for acquisitions, net	(7.8)	(7.8)
Purchase of property and equipment	(4.5)	(7.3)
Net cash used in investing activities	(12.3)	(15.1)
Cash flows from financing activities:
Net change in payable to lenders under loans with maturities 90 days or less	111.6	(98.0)
Proceeds from payables to lenders under loans with maturities greater than 90 days	311.5	0.0
Repayments of payables to lenders under loans with maturities greater than 90 days	(350.0)	0.0
Proceeds from issuance of senior secured term loan	21.5	175.0
Repayments of senior secured term loan	(4.9)	(2.2)
Repayments of note payable	(0.4)	(0.4)
Deferred payments on acquisitions	(0.2)	0.0
Repurchase of stock	(7.5)	0.0
Debt issuance costs	(1.5)	(3.2)
Exercise of stock options	4.0	0.8
Net cash (used in) provided by financing activities	84.1	72.0
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents	(2.7)	0.3
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents	1,239.1	(79.7)
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	3,690.4	2,110.4
Supplemental disclosure of cash flow information:
Cash paid for interest	64.3	69.8
Income taxes paid, net of cash refunds	13.2	11.3
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions	7.2	2.7
Assets acquired	352.6	25.9
Liabilities assumed	(352.0)	(7.3)
Total net assets acquired	$ 0.6	$ 18.6