Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES:
Consolidated net income (loss)	$ 268,318	$ (3,839)	$ 173,755
Adjustments to reconcile consolidated net income to net cash provided by operating activities:
Fixed asset depreciation and intangible asset amortization	81,997	44,747	47,152
Employee loan amortization and reserves on employee loans	86,708	25,708	194,996
Equity-based compensation and allocations of net income to limited partnership units and FPUs	278,684	150,125	433,998
Deferred compensation expense	23,189
(Gains) losses on equity method investments	(1,863)	8,621	9,508
Accretion of discount on convertible notes	5,835	4,852	5,157
Unrealized (gain) loss on marketable securities	(27,103)	(8,633)	(7,668)
Impairment of fixed assets	19,127	4,193	6,101
Deferred tax provision (benefit)	60,015	(26,185)	(81,989)
Sublease provision adjustment	2,019	31	318
Recognition of earn-out and related hedges	(52,917)	(42,091)	(31,861)
Cumulative realized gain on marketable securities (see Note 10 - "Marketable Securities")	(35,994)
Gain on divestitures and sale of investments	(401,902)		(550,759)
Forfeitures of Class A common stock	(1,701)	(2,452)
Other	(5,491)		1,508
Consolidated net income, adjusted for non-cash and non-operating items	298,921	155,077	200,216
Decrease (increase) in operating assets:
Receivables from broker-dealers, clearing organizations, customers and related broker-dealers	538,881	(292,065)	(51,710)
Loans, forgivable loans and other receivables from employees and partners, net	(100,691)	(13,616)	(49,478)
Accrued commissions receivable, net	27,550	7,265	(49,542)
Securities borrowed	62,736	(62,736)
Securities owned	4,248	611	(1,213)
Receivables from related parties	(3,291)	(1,096)	(5,362)
Cash segregated under regulatory requirements	9,104	(3,457)	(5,295)
Other assets	25,686	(25,350)	(73,422)
Increase (decrease) in operating liabilities:
Payables to broker-dealers, clearing organizations, customers and related broker-dealers	(577,136)	342,998	48,973
Payables to related parties	(1,065)	8,745	(24,862)
Securities sold, not yet purchased	(1,545)	(2,031)	2,031
Securities loaned	117,890
Accounts payable, accrued and other liabilities	(87,764)	51,832	145,167
Accrued compensation	(100,660)	17,366	30,863
Net cash provided by operating activities	212,864	183,543	166,366
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of fixed assets	(37,626)	(13,727)	(22,988)
Capitalization of software development costs	(18,496)	(12,668)	(15,000)
Purchase of equity method investments	(1,268)	(7,392)	(1,748)
Proceeds from sale of marketable securities	129,183	42,999
Payments for acquisitions, net of cash acquired	(173,072)	(129,979)	(746)
Proceeds from divestitures, net	14,485		575,287
Purchase of marketable securities	(11,630)	(72,911)	(5,361)
Purchase of exchange membership			(1,696)
Capitalization of trademarks, patent defense and registration costs	(845)	(578)	(995)
Net cash (used in) provided by investing activities	(99,269)	(194,256)	526,753
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayments of collateralized borrowings	(4,976)	(1,599)	(48,246)
Issuance of collateralized borrowings, net of deferred issuance costs	27,918	295,091
Earnings distributions	(73,765)	(58,789)	(98,345)
Redemption and repurchase of limited partnership interests	(34,287)	(81,331)	(113,345)
Dividends to stockholders	(131,360)	(105,132)	(91,395)
Repurchase of Class A common stock	(12,114)	(100,268)	(15,528)
Cancellation of restricted stock units in satisfaction of withholding tax requirements	(615)	(1,208)	(1,216)
Tax impact on delivery of equity awards			4,700
Proceeds from exercise of stock options	701
Proceeds from short-term borrowings	100,000
Repayments of short-term borrowings	(170,000)
Proceeds from sale of limited partnership interests	6,573
Other			32
Net cash used in financing activities	(291,925)	(53,236)	(363,343)
Effect of exchange rate changes on cash and cash equivalents	(8,740)	(4,693)	(1,266)
Net (decrease) increase in cash and cash equivalents	(187,070)	(68,642)	328,510
Cash and cash equivalents at beginning of period	648,277	716,919	388,409
Cash and cash equivalents at end of period	461,207	648,277	716,919
Supplemental cash information:
Cash paid during the period for taxes	43,661	87,928	152,105
Cash paid during the period for interest	59,281	32,099	33,175
Supplemental non-cash information:
Issuance of Class A common stock upon exchange of limited partnership interests	79,309	87,212	65,908
Issuance of Class A, contingent Class A common stock and limited partnership interests for acquisitions	39,635	57,907	1,776
Donations with respect to Charity Day			5,720
8.75% Convertible Notes [Member]
Supplemental non-cash information:
Issuance of Class A common stock upon conversion of 8.75% convertible notes	150,000
NASDAQ [Member]
Supplemental non-cash information:
Shares received	52,917	$ 42,091	$ 31,861
Trayport [Member]
Supplemental non-cash information:
Shares received	$ 629,084