Summary of Significant Accounting Policies - Principles of Consolidation (Details)	12 Months Ended
Dec. 31, 2017
Principles of Consolidation
Ownership percentage for equity method investments	20.00%
Interests in variable interest entities (as a percent)	0.00%
Maximum
Principles of Consolidation
Ownership percentage for equity method investments	50.00%
Ownership percentage for consolidation of affiliate	50.00%
Minimum
Principles of Consolidation
Ownership percentage for equity method investments	20.00%

Represents the maximum percentage of the outstanding voting shares of another entity held directly or indirectly by the parent for consolidation of financial statements.

No definition available.

+ Details

Name:	itg_OwnershipPercentageByParentForConsolidation
Namespace Prefix:	itg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

+ Details

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Balance Type:	na
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- Definition

Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64801991&loc=SL6759159-111685

+ Details

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Period Type:	duration

- Details

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- Details

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