Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41242-110953

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(10))
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-Name Accounting Standards Codification
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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Number of businesses the entity is managed and operated as.

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Geographic Information and Business Segments - Segments (Details) $ in Thousands		3 Months Ended
	Jul. 19, 2017 business segment	Mar. 31, 2018 USD ($)	Sep. 30, 2017 segment	Mar. 31, 2017 USD ($)
Segment reporting
Number of businesses Company is managed and operated as \| business	1
Number of reportable segments \| segment	1
Number of operating segments \| segment			3
Revenues		$ 815,053		$ 147,287
Income (loss) before income taxes and noncontrolling interest		468,536		23,882
Market Making
Segment reporting
Revenues		432,536		144,448
Income (loss) before income taxes and noncontrolling interest		163,163		23,465
Execution Services
Segment reporting
Revenues		383,779		2,779
Income (loss) before income taxes and noncontrolling interest		331,185		489
Corporate
Segment reporting
Revenues		(1,262)		60
Income (loss) before income taxes and noncontrolling interest		$ (25,812)		$ (72)