Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Assets
Financial instruments owned, at fair value	$ 2,068,734	$ 1,848,806
Financial instruments owned, at fair value, Counterparty and Cash Collateral Netting	(211,398)	(2,790,242)
Financial instruments owned, pledged as collateral	696,956	791,115
Other Assets	48,966	48,273
Liabilities
Financial instruments sold, not yet purchased, at fair value	2,497,958	2,475,395
Financial instruments sold, not yet purchased, at fair value, Counterparty and Cash Collateral Netting	(196,535)	(2,719,954)
Equity securities
Assets
Financial instruments owned, at fair value	1,680,777	1,609,901
Financial instruments owned, pledged as collateral	654,366	748,846
Liabilities
Financial instruments sold, not yet purchased, at fair value	2,186,702	2,268,330
U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	127,537	106,013
Financial instruments owned, pledged as collateral		0
Liabilities
Financial instruments sold, not yet purchased, at fair value	41,804	115,112
Corporate Bonds
Assets
Financial instruments owned, at fair value	171,591	87,500
Liabilities
Financial instruments sold, not yet purchased, at fair value	244,700	40,123
Exchange traded notes
Assets
Financial instruments owned, at fair value	49,137	31,362
Financial instruments owned, pledged as collateral	42,590	42,269
Liabilities
Financial instruments sold, not yet purchased, at fair value	21,646	39,984
Equity investment
Assets
Other Assets	46,245	45,856
Exchange stock
Assets
Other Assets	2,721	2,417
Currency forwards
Assets
Financial instruments owned, at fair value	31,154	2,131
Financial instruments owned, at fair value, Counterparty and Cash Collateral Netting	(211,398)	(2,790,242)
Liabilities
Financial instruments sold, not yet purchased, at fair value	19	795
Financial instruments sold, not yet purchased, at fair value, Counterparty and Cash Collateral Netting	(196,535)	(2,719,954)
Options
Assets
Financial instruments owned, at fair value	8,538	11,899
Liabilities
Financial instruments sold, not yet purchased, at fair value	3,087	11,051
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	715,730	694,441
Financial instruments owned, pledged as collateral	362,451	396,778
Other Assets	2,721	2,417
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,065,007	1,055,472
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Assets
Financial instruments owned, at fair value	600,259	587,680
Financial instruments owned, pledged as collateral	362,439	389,810
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,022,814	931,992
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	106,690	91,466
Financial instruments owned, pledged as collateral		0
Liabilities
Financial instruments sold, not yet purchased, at fair value	39,091	112,058
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate Bonds
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange traded notes
Assets
Financial instruments owned, at fair value	243	3,396
Financial instruments owned, pledged as collateral	12	6,968
Liabilities
Financial instruments sold, not yet purchased, at fair value	15	371
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investment
Assets
Other Assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Exchange stock
Assets
Other Assets	2,721	2,417
Quoted Prices in Active Markets for Identical Assets (Level 1) | Currency forwards
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Options
Assets
Financial instruments owned, at fair value	8,538	11,899
Liabilities
Financial instruments sold, not yet purchased, at fair value	3,087	11,051
Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	1,564,402	3,944,607
Financial instruments owned, pledged as collateral	334,505	394,337
Other Assets	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,629,486	4,139,877
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Financial instruments owned, at fair value	1,080,518	1,022,221
Financial instruments owned, pledged as collateral	291,927	359,036
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,163,888	1,336,338
Significant Other Observable Inputs (Level 2) | U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	20,847	14,547
Financial instruments owned, pledged as collateral		0
Liabilities
Financial instruments sold, not yet purchased, at fair value	2,713	3,054
Significant Other Observable Inputs (Level 2) | Corporate Bonds
Assets
Financial instruments owned, at fair value	171,591	87,500
Liabilities
Financial instruments sold, not yet purchased, at fair value	244,700	40,123
Significant Other Observable Inputs (Level 2) | Exchange traded notes
Assets
Financial instruments owned, at fair value	48,894	27,966
Financial instruments owned, pledged as collateral	42,578	35,301
Liabilities
Financial instruments sold, not yet purchased, at fair value	21,631	39,613
Significant Other Observable Inputs (Level 2) | Equity investment
Assets
Other Assets	0	0
Significant Other Observable Inputs (Level 2) | Exchange stock
Assets
Other Assets	0	0
Significant Other Observable Inputs (Level 2) | Currency forwards
Assets
Financial instruments owned, at fair value	242,552	2,792,373
Liabilities
Financial instruments sold, not yet purchased, at fair value	196,554	2,720,749
Significant Other Observable Inputs (Level 2) | Options
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3)
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned, pledged as collateral	0	0
Other Assets	46,245	45,856
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3) | Equity securities
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned, pledged as collateral	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3) | U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned, pledged as collateral		0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3) | Corporate Bonds
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3) | Exchange traded notes
Assets
Financial instruments owned, at fair value	0	0
Financial instruments owned, pledged as collateral	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3) | Equity investment
Assets
Other Assets	46,245	45,856
Significant Unobservable Inputs (Level 3) | Exchange stock
Assets
Other Assets	0	0
Significant Unobservable Inputs (Level 3) | Currency forwards
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Significant Unobservable Inputs (Level 3) | Options
Assets
Financial instruments owned, at fair value	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	$ 0	$ 0