Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income (loss)	$ 128,008	$ 260,115	$ (14,413)
Adjustments to reconcile consolidated net income (loss) to net cash provided by (used in) operating activities:
Fixed asset depreciation and intangible asset amortization	75,898	83,508	46,124
Employee loan amortization and reserves on employee loans	55,799	86,725	25,708
Equity-based compensation and allocations of net income to limited partnership units and FPUs	216,874	278,684	150,125
Deferred compensation expense	15,974	23,189
Losses (gains) on equity method investments	(3,543)	(2,597)	7,969
Amortization of discount (premium) on notes payable	(4,021)	5,835	4,852
Unrealized loss (gain) on marketable securities	(359)	(27,103)	(8,633)
Impairment of fixed assets and intangible assets	4,383	19,128	4,193
Deferred tax provision (benefit)	(33,245)	60,015	(26,185)
Sublease provision adjustment	(807)	2,019	31
Recognition of earn-out and related hedges	(67,016)	(52,917)	(42,091)
Realized losses (gains) on marketable securities	(13,410)	(35,994)
Change in estimated acquisition earn-out payables	(13,751)	74	(3,291)
Loss (gains) on sale of cost method investments	(7,051)
Loss (gains) on divestitures		(401,902)
Forfeitures of Class A common stock	(374)	(1,701)	(2,452)
Other	(391)	(5,491)
Consolidated net income (loss), adjusted for non-cash and non-operating items	352,968	291,587	141,937
Consolidated net income (loss), adjusted for non-cash and non-operating items	352,968	291,587	141,937
Decrease (increase) in operating assets:
Cash segregated under regulatory requirements	(3,694)	9,104	(3,457)
Reverse repurchase agreements	(54,659)
Securities owned	(2,996)	4,248	611
Securities borrowed		62,736	(62,736)
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers	322,960	539,042	(292,354)
Accrued commissions receivable, net	541	27,550	7,265
Loans, forgivable loans and other receivables from employees and partners, net	(137,516)	(100,726)	(13,616)
Receivables from related parties	3,067	3,839	(2,401)
Other assets	12,446	25,643	(25,019)
Increase (decrease) in operating liabilities:
Securities loaned	(117,967)	117,890
Securities sold, not yet purchased		(1,545)	(2,031)
Accrued compensation	(59,170)	(100,686)	17,392
Payables to broker-dealers, clearing organizations, customers and related broker-dealers	(340,558)	(577,136)	342,998
Payables to related parties	6,431	(356)	2,163
Accounts payable, accrued and other liabilities	(5,817)	(74,101)	56,081
Net cash provided by (used in) operating activities	(23,964)	227,089	166,833
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of fixed assets	(52,659)	(38,546)	(14,067)
Capitalization of software development costs	(24,956)	(18,496)	(12,668)
Purchase of equity method investments	(1,235)	(1,268)	(892)
Proceeds from equity method investments	1,080
Payments for acquisitions, net of cash acquired	(162,265)	(173,072)	(129,979)
Proceeds from divestitures, net		14,485
Purchase of marketable securities	(68,396)	(11,630)	(72,911)
Proceeds from sale of marketable securities	635,646	129,183	42,999
Proceeds from sale of cost method investments	7,106
Capitalization of trademarks, patent defense and registration costs	(546)	(845)	(577)
Net cash provided by (used in) investing activities	333,775	(100,189)	(188,095)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayments of collateralized borrowings	(6,858)	(4,976)	(1,599)
Repayments of convertible notes	(159,932)
Issuance of senior notes, net of deferred issuance costs	295,768		295,091
Issuance of collateralized borrowings, net of deferred issuance costs		27,918
Earnings distributions	(69,740)	(73,765)	(58,789)
Redemption and repurchase of limited partnership interests	(51,992)	(34,287)	(81,331)
Dividends to stockholders	(170,795)	(131,360)	(105,132)
Repurchase of Class A common stock	(96,025)	(12,114)	(100,268)
Cancellation of restricted stock units in satisfaction of withholding tax requirements	(1,542)	(615)	(1,208)
Proceeds from issuance of Class A common stock, net of costs	15,280
Deemed contribution due to acquisition of Lucera	15,005		12,158
Acquisition of Lucera	(29,037)
Proceeds from exercise of stock options	421	701
Proceeds from short-term borrowings		100,000
Repayments of short-term borrowings		(170,000)
Proceeds from sale of limited partnership interests	2,357	6,573
Payments on acquisition earn-outs	(12,949)	(13,791)	(250)
Net cash provided by (used in) financing activities	(270,039)	(305,716)	(41,328)
Effect of exchange rate changes on cash and cash equivalents	118	(8,740)	(4,693)
Net increase (decrease) in cash and cash equivalents	39,890	(187,556)	(67,283)
Cash and cash equivalents at beginning of period	462,134	649,690	716,973
Cash and cash equivalents at end of period	502,024	462,134	649,690
Supplemental cash information:
Cash paid during the period for taxes	94,678	43,661	87,928
Cash paid during the period for interest	63,602	59,281	32,099
Supplemental non-cash information:
Issuance of Class A common stock upon exchange of limited partnership interests	75,423	79,309	87,212
Issuance of Class A and contingent Class A common stock and limited partnership interests for acquisitions	20,930	39,635	57,907
8.75% Convertible Notes [Member]
Supplemental non-cash information:
Issuance of Class A common stock upon conversion of convertible notes	68	150,000
NASDAQ [Member]
Supplemental non-cash information:
Shares received	$ 67,016	52,917	$ 42,091
Trayport [Member]
Supplemental non-cash information:
Shares received		$ 629,084