Condensed Consolidated Statement of Changes in Stockholders' Equity (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Consolidated Statement of Changes in Stockholders' Equity
Issuance of common stock for restricted share awards, shares	1,029,520	1,029,623	670,933
Issuance of common stock for employee stock unit awards, shares	14,228		71,580
Increase in current taxes payable	$ 0.6	$ 1.5	$ 4.1
Awards reclassified to liability for cash settlement, shares			259,840
Issuance of common stock for the employee stock purchase plan, shares	71,587	121,363	137,782
Shares Repurchased Under Board Authorization	2,671,637us-gaap_TreasuryStockSharesAcquired	2,005,200us-gaap_TreasuryStockSharesAcquired	2,470,000us-gaap_TreasuryStockSharesAcquired
Shares repurchased to satisfy minimum statutory withholding tax upon the net settlement of equity awards	367,367	352,051	270,467

The number of shares for which the value is classified to liability for cash settlement.

No definition available.

Name:	itg_AwardsClassifiedToLiabilityForCashSettlementShares
Namespace Prefix:	itg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Issuance of common stock for employee stock unit awards.

+ References

No definition available.

+ Details

Name:	itg_StockIssuedDuringPeriodSharesStockUnitAwards
Namespace Prefix:	itg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase in additional paid in capital (APIC) resulting from a tax benefit associated with share-based compensation plan other than an employee stock ownership plan (ESOP). Includes, but is not limited to, excess tax benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 740

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6419406&loc=d3e23524-113945

+ Details

Name:	us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

For net-share settlement of share-based awards when the employer settles employees' income tax withholding obligations, this element represents the number of shares the employees use to repay the employer.

+ References

No definition available.

+ Details

Name:	us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_StatementOfStockholdersEquityAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

-Article 5

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares issued during the period related to Restricted Stock Awards, net of any shares forfeited.

+ References

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

+ Details

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration