Condensed Consolidated Cash Flows Statements - USD ($)	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 16,300,000	$ 18,200,000
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	3,900,000	2,900,000
Bad debts	0	300,000
Recovery of bad debt on physical coal	0	(2,400,000)
Deferred income taxes	(600,000)	0
Amortization of debt issuance costs	400,000	200,000
Amortization of share-based compensation	2,600,000	1,900,000
Changes in operating assets and liabilities, net:
Securities and other assets segregated under federal and other regulations	299,000,000	330,300,000
Securities purchased under agreements to resell	(178,600,000)	(320,500,000)
Securities borrowed	(4,500,000)	(737,900,000)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties, net	281,100,000	(225,500,000)
Receivables from clients, net	93,300,000	(63,700,000)
Notes receivable, net	(2,900,000)	2,200,000
Income taxes receivable	500,000	100,000
Financial instruments owned, at fair value	(2,200,000)	300,000
Physical commodities inventory, net	(20,400,000)	0
Operating right of use assets	1,500,000	0
Other assets	(4,100,000)	(6,100,000)
Accounts payable and other accrued liabilities	(25,800,000)	(15,700,000)
Operating lease liabilities	(1,300,000)	0
Payables to clients	114,200,000	(361,200,000)
Payables to broker-dealers, clearing organizations and counterparties	14,400,000	133,600,000
Income taxes payable	400,000	1,000,000
Securities sold under agreements to repurchase	157,900,000	302,600,000
Securities loaned	(29,100,000)	753,100,000
Financial instruments sold, not yet purchased, at fair value	(48,400,000)	(26,700,000)
Net cash provided by (used in) operating activities	667,600,000	(213,000,000)
Cash flows from investing activities:
Cash paid for acquisitions, net	(5,100,000)	(700,000)
Purchase of property and equipment	(1,800,000)	(4,500,000)
Net cash used in investing activities	(6,900,000)	(5,200,000)
Cash flows from financing activities:
Net change in payable to lenders under loans with maturities 90 days or less	(106,000,000)	80,400,000
Proceeds from payables to lenders under loans with maturities greater than 90 days	180,500,000	0
Repayments of payables to lenders under loans with maturities greater than 90 days	(138,000,000)	0
Proceeds from issuance of senior secured term loan	21,500,000	0
Repayments of senior secured term loan	(2,500,000)	0
Repayments of note payable	(100,000)	(200,000)
Repurchase of common stock	(400,000)	0
Debt issuance costs	(200,000)	(900,000)
Exercise of stock options	1,500,000	300,000
Net cash (used in) provided by financing activities	(43,700,000)	79,600,000
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents	700,000	300,000
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents	617,700,000	(138,300,000)
Cash, segregated cash, cash equivalents, and segregated cash equivalents at beginning of period	2,451,300,000	2,190,100,000
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	3,069,000,000	2,051,800,000
Supplemental disclosure of cash flow information:
Cash paid for interest	33,000,000	33,700,000
Income taxes paid, net of cash refunds	5,200,000	5,300,000
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions	4,400,000	0
Assets acquired	296,500,000	3,100,000
Liabilities assumed	(295,800,000)	(900,000)
Total net assets acquired	$ 700,000	$ 2,200,000