Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Financial Condition (Parent Only) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Cash and cash equivalents	$ 632,234	$ 581,313	$ 578,768	$ 674,281
Fixed assets and leasehold improvements, at cost, less accumulated depreciation and amortization	151,645	94,858
Goodwill and Intangible assets, less accumulated amortization	100,338	100,471
Deferred tax asset, net	109,861	151,225
Other assets	164,168	222,831
Total assets	6,261,287	6,040,535	6,821,375
Liabilities
Accrued compensation expense	132,406	154,547
Accrued expenses and other liabilities	156,828	134,026
Income taxes payable	71,391	0
Debt	454,353	484,989
Total liabilities	4,904,004	4,596,437
Total equity	1,357,283	1,444,098	1,522,577	1,509,537
Total liabilities and equity	6,261,287	6,040,535
Parent Company
Assets
Cash and cash equivalents	369,882	333,982	$ 302,700	$ 253,017
Receivable from subsidiaries	0	212,336
Investments in subsidiaries	1,509,526	1,039,250
Fixed assets and leasehold improvements, at cost, less accumulated depreciation and amortization	88,822	2,755
Goodwill and Intangible assets, less accumulated amortization	0	218
Deferred tax asset, net	87,847	76,747
Subordinated loans to subsidiaries	300,000	280,000
Other assets	13,181	41,178
Total assets	2,369,258	1,986,466
Liabilities
Accrued compensation expense	13,812	21,775
Payable to subsidiaries	405,414	0
Accrued expenses and other liabilities	64,279	35,604
Income taxes payable	74,117	0
Debt	454,353	484,989
Total liabilities	1,011,975	542,368
Total equity	1,357,283	1,444,098
Total liabilities and equity	$ 2,369,258	$ 1,986,466