Additional Interest Percent Obligation From Guarantor if Certain Obligations Are Not Met

No definition available.

Name:	kcg_AdditionalInterestPercentObligationFromGuarantorifCertainObligationsAreNotMet
Namespace Prefix:	kcg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Repurchase Amount, Interest Expense

+ References

No definition available.

+ Details

Name:	kcg_DebtInstrumentRepurchaseAmountInterestExpense
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Deposit Cash Made to Credit Facilty to Repurchase Convertible Notes

+ References

No definition available.

+ Details

Name:	kcg_DepositCashMadetoCreditFaciltytoRepurchaseConvertibleNotes
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Escrow Agent Fees and Expenses

+ References

No definition available.

+ Details

Name:	kcg_EscrowAgentFeesandExpenses
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

increase in Line of Credit Facility, Maximum Borrowing Capacity

+ References

No definition available.

+ Details

Name:	kcg_IncreaseinLineofCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Loan Guarantee Percent

+ References

No definition available.

+ Details

Name:	kcg_LoanGuaranteePercent
Namespace Prefix:	kcg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of price equal to principal amount of notes.

+ References

No definition available.

+ Details

Name:	kcg_PercentageOfPriceEqualToPrincipalAmountOfNotes
Namespace Prefix:	kcg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, the carrying value of convertible subordinated debt, as of the balance sheet date, initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. This form of debt can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder, and places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX210.5-02.22))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Article 5

-Subsection 20, 22

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

+ Details

Name:	us-gaap_ConvertibleSubordinatedDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Interest rate stated in the contractual debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of the required periodic payments applied to principal.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

+ Details

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

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRepurchaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

+ References

No definition available.

+ Details

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

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The effective interest rate at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 10

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

Subsequent Events (Details) (USD $)	6 Months Ended			1 Months Ended			1 Months Ended											1 Months Ended
Subsequent Events (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012	Mar. 31, 2010	Jul. 31, 2013 Subsequent Event [Member]	Jul. 31, 2013 Subsequent Event [Member] Secured Debt [Member] Jefferies Finance LLC and Goldman Sachs Bank USA [Member]	Jun. 30, 2013 Subsequent Event [Member] Secured Debt [Member] Jefferies Finance LLC and Goldman Sachs Bank USA [Member]	Jul. 31, 2013 Subsequent Event [Member] Secured Debt [Member] OCTEG-KCA [Member]	Jul. 01, 2013 Subsequent Event [Member] Secured Debt [Member] OCTEG-KCA [Member]	Jun. 30, 2013 Subsequent Event [Member] Revolving Credit Facility [Member] Jefferies Finance LLC and Goldman Sachs Bank USA [Member]	Jul. 01, 2013 Subsequent Event [Member] Revolving Credit Facility [Member] OCTEG-KCA [Member]	Jul. 01, 2013 Subsequent Event [Member] Swingline Facilty [Member] OCTEG-KCA [Member]	Jun. 30, 2013 Subsequent Event [Member] Prime Rate [Member] Secured Debt [Member] Jefferies Finance LLC and Goldman Sachs Bank USA [Member]	Jun. 30, 2013 Subsequent Event [Member] LIBOR [Member] Secured Debt [Member] Jefferies Finance LLC and Goldman Sachs Bank USA [Member]	Jun. 30, 2013 Subsequent Event [Member] LIBOR Floor [Member] Secured Debt [Member] Jefferies Finance LLC and Goldman Sachs Bank USA [Member]	Jun. 30, 2013 Subsequent Event [Member] Borrowing Base A [Member] Revolving Credit Facility [Member] OCTEG-KCA [Member]	Jun. 30, 2013 Subsequent Event [Member] Borrowing Base B Loan [Member] Revolving Credit Facility [Member] OCTEG-KCA [Member]	Aug. 05, 2013 Convertible Debt [Member] Subsequent Event [Member]	Jun. 30, 2013 Secured Debt [Member] OCTEG-KCA [Member] Subsequent Event [Member]	Jun. 05, 2013 Secured Debt [Member] OCTEG-KCA [Member] Subsequent Event [Member]
Subsequent Event [Line Items]
Term Credit Agreement, Carrying Amount	$ 0	$ 100,000,000				$ 535,000,000		$ 450,000,000											$ 305,000,000
Revolving credit facility amount									50,000,000	750,000,000	50,000,000
increase in line of credit facility, maximum borrowing capacity										300,000,000
Interest rate of credit agreement												2.25%	1.00%	1.25%
Percentage points added to interest rate base						4.50%						0.50%	3.50%		1.75%	2.25%
Principal payments in 2014						235,000,000
Periodic princpal payment					7,500,000
Loan guarantee percent						100.00%
Interest rate																			8.25%
Commitment fee percentage of average daily amount unused portion of revolving credit agreement	0.25%						0.35%
Debt issuance cost							500,000
Escrow agent fees and expenses																		3,000,000
Additional interest percent obligation from guarantor if certain obligations are not met																		1.00%
Issue of cash convertible senior subordinated notes	375,000,000		375,000,000
Percentage of price equal to principal amount of notes	100.00%
Deposit cash made to credit facilty to repurchase convertible notes				375,000,000
Debt instrument, repurchase amount																	249,200,000
Debt instrument, interest expense associated with repurchase amount																	$ 3,400,000