Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Assets
Financial instruments owned, at fair value	$ 3,238,995	$ 2,369,192
Financial instruments owned, at fair value, counterparty and cash collateral netting	(206,125)	(291,964)
Financial instruments owned, pledged as collateral	1,017,960	746,539
Other Assets	84,378	68,316
Liabilities
Financial instruments sold, not yet purchased, at fair value	3,510,779	2,923,708
Financial instruments sold, not yet purchased, at fair value, counterparty and cash collateral netting	(208,356)	(292,870)
Payables to broker-dealers and clearing organizations	571,526	876,446
Interest rate swap
Liabilities
Payables to broker-dealers and clearing organizations	21,037	63,513
Equity securities
Assets
Financial instruments owned, at fair value	2,273,037	1,955,589
Financial instruments owned, pledged as collateral	1,012,569	734,024
Liabilities
Financial instruments sold, not yet purchased, at fair value	2,290,017	2,445,050
U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	355,869	199,782
Liabilities
Financial instruments sold, not yet purchased, at fair value	340,720	103,157
Corporate Bonds
Assets
Financial instruments owned, at fair value	598,944	135,518
Liabilities
Financial instruments sold, not yet purchased, at fair value	851,871	358,734
Exchange traded notes
Assets
Financial instruments owned, at fair value	2,469	19,827
Financial instruments owned, pledged as collateral	5,391	12,515
Liabilities
Financial instruments sold, not yet purchased, at fair value	22,962	7,431
Equity investment
Assets
Other Assets	81,358	66,030
Exchange stock
Assets
Other Assets	3,020	2,286
Currency forwards
Assets
Financial instruments owned, at fair value	133	49,396
Financial instruments owned, at fair value, counterparty and cash collateral netting	(206,125)	(291,964)
Liabilities
Financial instruments sold, not yet purchased, at fair value	1	95
Financial instruments sold, not yet purchased, at fair value, counterparty and cash collateral netting	(208,356)	(292,870)
Options
Assets
Financial instruments owned, at fair value	8,543	9,080
Liabilities
Financial instruments sold, not yet purchased, at fair value	5,208	9,241
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Financial instruments owned, at fair value	918,470	922,393
Financial instruments owned, pledged as collateral	670,277	496,945
Other Assets	3,020	2,286
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,818,359	1,399,496
Payables to broker-dealers and clearing organizations	235	3,517
Quoted Prices in Active Markets for Identical Assets (Level 1)  | Interest rate swap
Liabilities
Payables to broker-dealers and clearing organizations	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1)  | Equity securities
Assets
Financial instruments owned, at fair value	572,567	761,484
Financial instruments owned, pledged as collateral	670,277	496,943
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,482,386	1,307,082
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	337,350	151,723
Liabilities
Financial instruments sold, not yet purchased, at fair value	330,765	83,173
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	10	106
Financial instruments owned, pledged as collateral	0	2
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
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	3,020	2,286
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	8,543	9,080
Liabilities
Financial instruments sold, not yet purchased, at fair value	5,208	9,241
Significant Other Observable Inputs (Level 2)
Assets
Financial instruments owned, at fair value	2,526,650	1,738,763
Financial instruments owned, pledged as collateral	347,683	249,594
Other Assets	0	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,900,776	1,817,082
Payables to broker-dealers and clearing organizations	571,291	872,929
Significant Other Observable Inputs (Level 2)  | Interest rate swap
Liabilities
Payables to broker-dealers and clearing organizations	21,037	63,513
Significant Other Observable Inputs (Level 2)  | Equity securities
Assets
Financial instruments owned, at fair value	1,700,470	1,194,105
Financial instruments owned, pledged as collateral	342,292	237,081
Liabilities
Financial instruments sold, not yet purchased, at fair value	807,631	1,137,968
Significant Other Observable Inputs (Level 2)  | U.S. and Non-U.S. government obligations
Assets
Financial instruments owned, at fair value	18,519	48,059
Liabilities
Financial instruments sold, not yet purchased, at fair value	9,955	19,984
Significant Other Observable Inputs (Level 2)  | Corporate Bonds
Assets
Financial instruments owned, at fair value	598,944	135,518
Liabilities
Financial instruments sold, not yet purchased, at fair value	851,871	358,734
Significant Other Observable Inputs (Level 2)  | Exchange traded notes
Assets
Financial instruments owned, at fair value	2,459	19,721
Financial instruments owned, pledged as collateral	5,391	12,513
Liabilities
Financial instruments sold, not yet purchased, at fair value	22,962	7,431
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	206,258	341,360
Liabilities
Financial instruments sold, not yet purchased, at fair value	208,357	292,965
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	81,358	66,030
Liabilities
Financial instruments sold, not yet purchased, at fair value	0	0
Payables to broker-dealers and clearing organizations	0	0
Significant Unobservable Inputs (Level 3)  | Interest rate swap
Liabilities
Payables to broker-dealers and clearing organizations	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
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	81,358	66,030
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