Subsequent Event Subsequent Event	12 Months Ended
Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event The Company has evaluated subsequent events through the date the Consolidated Financial Statements were issued. The Company did not note any subsequent events requiring adjustment or disclosure in the Consolidated Financial Statements except for the following. In January 2015, the Company announced the sale of KCG Hotspot to BATS. Under the terms of the agreement, the Company will receive $365.0 million in cash upon the close. In addition, the Company and BATS have agreed to share certain tax benefits, which could result in further payments to the Company of up to approximately $70.0 million in the three-year period following the close. The transaction, which is subject to customary closing conditions, is expected to close in March 2015. The Company does not believe that this sale represents a strategic shift that will have a major effect on the Company’s operations and financial results, and therefore KCG Hotspot does not meet the requirements to be treated as a discontinued operation. The sale of KCG Hotspot is structured as an asset sale for income tax purposes and BATS and KCG have agreed to share certain related tax benefits that potentially accrue to BATS after the closing of the transaction. KCG will share in 70% of the actual tax benefits to BATS for the first three years after the closing and 50% of the actual tax benefits thereafter (the “Annual Tax Benefits”). However, KCG has a one-time option exercisable within 30 days of the third anniversary of the closing of the transaction to terminate the continued tax sharing arrangement in exchange for a one-time payment of $50.0 million, which BATS has the right to exercise after KCG’s option expires. However, the receipt of the Annual Tax Benefits by KCG is subject to BATS having sufficient net income to receive the tax benefits. BATS’s net income could decrease due to numerous factors, which are outside of the control of KCG. In addition, any decrease in the corporate tax rates applicable to BATS could reduce the size or certainty of the Annual Tax Benefits. In addition, KCG will not be entitled to the Annual Tax Benefits in the first three years if, during the five month period beginning on January 27, 2015 (subject to extension in limited cases) (the “Measurement Period”), the average daily notional FX trading volume on KCG Hotspot for any 60 trading day rolling period is less than or equal to a threshold set at 70% of the average daily notional FX trading volume on KCG Hotspot for the month with the lowest trading volume over the last two calendar years (the “Trigger Event”) and, after the Trigger Event but during the Measurement Period, the average daily notional FX trading volume on Hotspot for any 15 trading day rolling period is not 80% or more of the average daily notional FX trading volume on Hotspot for the 2014 calendar year and no subsequent Trigger Event occurs during the Measurement Period.

Subsequent Event

The Company has evaluated subsequent events through the date the Consolidated Financial Statements were issued. The Company did not note any subsequent events requiring adjustment or disclosure in the Consolidated Financial Statements except for the following.

In January 2015, the Company announced the sale of KCG Hotspot to BATS. Under the terms of the agreement, the Company will receive $365.0 million in cash upon the close. In addition, the Company and BATS have agreed to share certain tax benefits, which could result in further payments to the Company of up to approximately $70.0 million in the three-year period following the close.

The transaction, which is subject to customary closing conditions, is expected to close in March 2015. The Company does not believe that this sale represents a strategic shift that will have a major effect on the Company’s operations and financial results, and therefore KCG Hotspot does not meet the requirements to be treated as a discontinued operation.

The sale of KCG Hotspot is structured as an asset sale for income tax purposes and BATS and KCG have agreed to share certain related tax benefits that potentially accrue to BATS after the closing of the transaction. KCG will share in 70% of the actual tax benefits to BATS for the first three years after the closing and 50% of the actual tax benefits thereafter (the “Annual Tax Benefits”). However, KCG has a one-time option exercisable within 30 days of the third anniversary of the closing of the transaction to terminate the continued tax sharing arrangement in exchange for a one-time payment of $50.0 million, which BATS has the right to exercise after KCG’s option expires. However, the receipt of the Annual Tax Benefits by KCG is subject to BATS having sufficient net income to receive the tax benefits. BATS’s net income could decrease due to numerous factors, which are outside of the control of KCG. In addition, any decrease in the corporate tax rates applicable to BATS could reduce the size or certainty of the Annual Tax Benefits.

In addition, KCG will not be entitled to the Annual Tax Benefits in the first three years if, during the five month period beginning on January 27, 2015 (subject to extension in limited cases) (the “Measurement Period”), the average daily notional FX trading volume on KCG Hotspot for any 60 trading day rolling period is less than or equal to a threshold set at 70% of the average daily notional FX trading volume on KCG Hotspot for the month with the lowest trading volume over the last two calendar years (the “Trigger Event”) and, after the Trigger Event but during the Measurement Period, the average daily notional FX trading volume on Hotspot for any 15 trading day rolling period is not 80% or more of the average daily notional FX trading volume on Hotspot for the 2014 calendar year and no subsequent Trigger Event occurs during the Measurement Period.