Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Financial Condition (Parent Only) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 581,313	$ 578,768	$ 674,281		$ 427,631
Fixed assets and leasehold improvements, at cost, less accumulated depreciation and amortization	94,858	134,051
Goodwill and Intangible assets, less accumulated amortization	100,471	152,594
Deferred tax asset, net	151,225	154,759
Other assets	233,474	137,645
Total assets	6,051,178	6,830,654
Liabilities
Accrued compensation expense	154,547	114,559
Accrued expenses and other liabilities	134,026	143,677
Debt	495,632	422,259
Total liabilities	4,607,080	5,308,077
Total equity	1,444,098	1,522,577	1,509,537	$ 613,470	654,672
Total liabilities and equity	6,051,178	6,830,654
Parent Company
Assets
Cash and cash equivalents	333,982	302,700	$ 253,017		$ 114,511
Receivable from subsidiaries	212,336	21,851
Investments in subsidiaries	1,039,250	1,122,254
Fixed assets and leasehold improvements, at cost, less accumulated depreciation and amortization	2,755	0
Goodwill and Intangible assets, less accumulated amortization	218	0
Deferred tax asset, net	76,747	137,009
Subordinated loans to subsidiaries	280,000	280,000
Other assets	51,821	33,396
Total assets	1,997,109	1,897,210
Liabilities
Accrued compensation expense	21,775	13,208
Payable to subsidiaries	0	42,478
Accrued expenses and other liabilities	35,604	13,947
Debt	495,632	305,000
Total liabilities	553,011	374,633
Total equity	1,444,098	1,522,577
Total liabilities and equity	$ 1,997,109	$ 1,897,210