Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted (Loss) Earnings Per Computations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator / net income
Income from continuing operations and shares used in basic calculations	$ 62,434	$ 126,092	$ 16,151
Income from continuing operations and shares used in diluted calculations	62,434	126,092	16,151
Loss from continuing operations allocated to preferred and participating units	0	(21,565)	1,092
Income from continuing operations attributable to common shareholders	$ 62,434	$ 147,657	$ 15,059
Denominator / shares
Income from continuing operations and shares used in basic calculations (in shares)	112,854	80,143	48,970
Income from continuing operations and shares used in diluted calculations (in shares)	116,534	81,015	48,970
Basic earnings per share from continuing operations (in dollars per share)	$ 0.55	$ 1.84	$ 0.31
Diluted earnings per common share from continuing operations (in dollars per share)	$ 0.54	$ 1.82	$ 0.31
Restricted Awards [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	3,579	855	0
Stock Options and SARs [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	101	17	0

Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted (Loss) Earnings Per Computations (Detail) (USD $)
In Thousands, except Per Share data, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Numerator / net income

Income from continuing operations and shares used in basic calculations

$ 62,434us-gaap_IncomeLossFromContinuingOperations

$ 126,092us-gaap_IncomeLossFromContinuingOperations

$ 16,151us-gaap_IncomeLossFromContinuingOperations

Income from continuing operations and shares used in diluted calculations

62,434us-gaap_NetIncomeLossAttributableToParentDiluted

126,092us-gaap_NetIncomeLossAttributableToParentDiluted

16,151us-gaap_NetIncomeLossAttributableToParentDiluted

Loss from continuing operations allocated to preferred and participating units

0us-gaap_PreferredStockDividendsAndOtherAdjustments

(21,565)us-gaap_PreferredStockDividendsAndOtherAdjustments

1,092us-gaap_PreferredStockDividendsAndOtherAdjustments

Income from continuing operations attributable to common shareholders

$ 62,434kcg_NetIncomeLossAvailabletoCommonStockholdersBasicContinuingOperations

$ 147,657kcg_NetIncomeLossAvailabletoCommonStockholdersBasicContinuingOperations

$ 15,059kcg_NetIncomeLossAvailabletoCommonStockholdersBasicContinuingOperations

Denominator / shares

Income from continuing operations and shares used in basic calculations (in shares)

112,854us-gaap_WeightedAverageNumberOfSharesOutstandingBasic

80,143us-gaap_WeightedAverageNumberOfSharesOutstandingBasic

48,970us-gaap_WeightedAverageNumberOfSharesOutstandingBasic

Income from continuing operations and shares used in diluted calculations (in shares)

116,534us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding

81,015us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding

48,970us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding

Basic earnings per share from continuing operations (in dollars per share)

$ 0.55us-gaap_IncomeLossFromContinuingOperationsPerBasicShare

$ 1.84us-gaap_IncomeLossFromContinuingOperationsPerBasicShare

$ 0.31us-gaap_IncomeLossFromContinuingOperationsPerBasicShare

Diluted earnings per common share from continuing operations (in dollars per share)

$ 0.54us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare

$ 1.82us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare

$ 0.31us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare

Restricted Awards [Member]

Denominator / shares

Effect of dilutive stock based awards (in shares)

3,579us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember

855us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember

0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember

Stock Options and SARs [Member]

Denominator / shares

Effect of dilutive stock based awards (in shares)

101us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_AwardTypeAxis
= kcg_StockOptionsandStockAppreciationRightsSARsMember

17us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_AwardTypeAxis
= kcg_StockOptionsandStockAppreciationRightsSARsMember

0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
/ us-gaap_AwardTypeAxis
= kcg_StockOptionsandStockAppreciationRightsSARsMember

- Details

Name:	kcg_DenominatorAbstract
Namespace Prefix:	kcg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Net Income (Loss) Available to Common Stockholders, Basic, Continuing Operations

+ References

No definition available.

+ Details

Name:	kcg_NetIncomeLossAvailabletoCommonStockholdersBasicContinuingOperations
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	kcg_NumeratorAbstract
Namespace Prefix:	kcg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 45

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4613673-111683

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.13)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

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

- Definition

The amount of net income (loss) from continuing operations per each share of common stock or unit outstanding during the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1252-109256

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 210.5-03.21)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.19)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.23)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 18

-Article 7

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 20

-Article 5

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 21

-Article 9

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsPerBasicShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of net income (loss) derived from continuing operations during the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1252-109256

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.21)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 18

-Article 7

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 20

-Article 5

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 21

-Article 9

+ Details

Name:	us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent, and includes adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 16

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1505-109256

+ Details

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

- Definition

The aggregate value of preferred stock dividends and other adjustments necessary to derive net income apportioned to common stockholders.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

+ Details

Name:	us-gaap_PreferredStockDividendsAndOtherAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

+ References

+ Details

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

- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

+ References

+ Details

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

- Definition

Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

+ References

+ Details

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

- Details

Name:	us-gaap_AwardTypeAxis=kcg_StockOptionsandStockAppreciationRightsSARsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: