Consolidated Cash Flows Statements - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 85,100,000	$ 55,500,000	$ 6,400,000
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
(Recovery) bad debt on physical coal	(2,400,000)	1,000,000	47,000,000
Depreciation and amortization	14,000,000	11,600,000	9,800,000
Provision for bad debts	2,500,000	3,100,000	4,300,000
Deferred income taxes	3,700,000	22,300,000	(9,800,000)
Amortization and extinguishment of debt issuance costs	1,500,000	1,000,000	1,900,000
Actuarial gain on pension and postretirement benefits	(300,000)	(300,000)	(300,000)
Amortization of share-based compensation expense	8,100,000	6,600,000	6,300,000
Gain on sale of property and equipment	0	0	(300,000)
Gain on acquisition	(5,500,000)	0	0
Changes in operating assets and liabilities, net:
Cash, securities and other assets segregated under federal and other regulations	337,200,000	(626,700,000)	570,800,000
Change in securities purchased under agreements to resell	(553,700,000)	(464,900,000)	203,000,000
Securities borrowed	(1,197,700,000)	(138,800,000)	(79,700,000)
Deposits with and receivables from broker-dealers, clearing organizations, and counterparties	(241,700,000)	(292,900,000)	186,300,000
Receivable from customers, net	(134,300,000)	(24,300,000)	(116,400,000)
Notes receivable, net	900,000	6,800,000	8,300,000
Income taxes receivable	(4,200,000)	(1,300,000)	500,000
Financial instruments owned, at fair value	(113,300,000)	(308,700,000)	(125,600,000)
Physical commodities inventory	3,000,000	(98,700,000)	(1,700,000)
Other assets	(8,300,000)	(3,300,000)	(16,000,000)
Accounts payable and other accrued liabilities	6,700,000	18,600,000	(19,600,000)
Payable to clients	(46,800,000)	520,000,000	290,900,000
Payable to broker-dealers, clearing organizations and counterparties	176,400,000	(27,800,000)	(124,100,000)
Income taxes payable	1,800,000	3,200,000	200,000
Securities sold under agreements to repurchase	837,000,000	543,700,000	226,000,000
Securities loaned	1,182,000,000	166,800,000	93,600,000
Financial instruments sold, not yet purchased, at fair value	(156,100,000)	153,900,000	(124,400,000)
Net cash provided by (used in) provided by operating activities	195,600,000	(473,600,000)	1,037,400,000
Cash flows from investing activities:
Cash paid for acquisitions, net	(28,900,000)	(3,700,000)	(6,000,000)
Purchase of exchange memberships and common stock	0	0	(200,000)
Sale of exchange memberships and common stock	0	800,000	0
Purchase of property and equipment	(11,900,000)	(12,500,000)	(16,100,000)
Net cash used in investing activities	(40,800,000)	(15,400,000)	(22,300,000)
Cash flows from financing activities:
Net change in lenders under loans with maturities 90 days or less	(162,600,000)	125,800,000	48,200,000
Proceeds from lenders under loans with maturities greater than 90 days	357,200,000	0	0
Repayments of lenders under loans with maturities greater than 90 days	(346,700,000)	0	0
Proceeds from issuance of senior secured term loan	175,000,000	0	0
Repayments of senior secured term loan	(6,600,000)	0	0
Repayment of senior notes	0	0	(45,500,000)
Payments of notes payable	(800,000)	(800,000)	(800,000)
Deferred payments on acquisitions	0	(5,500,000)	0
Share repurchase	(3,800,000)	0	0
Debt issuance costs	(3,300,000)	(400,000)	(300,000)
Exercise of stock options	1,200,000	2,600,000	3,400,000
Withholding taxes on stock option exercises	0	(800,000)	0
Income tax benefit on stock options and awards	0	0	700,000
Net cash provided by financing activities	9,600,000	120,900,000	5,700,000
Effect of exchange rates on cash, segregated cash, cash equivalents, and segregated cash equivalents	(700,000)	(4,100,000)	1,400,000
Net increase (decrease) in cash, segregated cash, cash equivalents, and segregated cash equivalents	163,700,000	(372,200,000)	1,022,200,000
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	2,287,600,000	2,659,800,000	1,637,600,000
Cash, segregated cash, cash equivalents, and segregated cash equivalents at end of period	2,451,300,000	2,287,600,000	2,659,800,000
Supplemental disclosure of cash flow information:
Cash paid for interest	153,200,000	78,900,000	38,000,000
Income taxes paid, net of cash refunds	24,600,000	22,200,000	17,100,000
Supplemental disclosure of non-cash investing and financing activities:
Identified intangible assets and goodwill on acquisitions	10,800,000	3,900,000	0
Additional consideration payable related to acquisitions	1,800,000	0	(200,000)
Acquisition of business:
Assets acquired	47,100,000	1,700,000	0
Liabilities acquired	(8,900,000)	(1,900,000)	0
Total net assets acquired	38,200,000	(200,000)	0
Escrow releases and deposits related to acquisitions	$ (4,200,000)	$ 0	$ (5,000,000)