Financial Assets and Liabilities - Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Assets
Cash restricted or segregated under regulations and other	$ 35,024	$ 56,662
Receivables from broker-dealers and clearing organizations	737,724	1,115,185
Receivables from customers	106,245	80,830
Other assets	303,720	303,916
Liabilities
Payables to broker-dealers and clearing organizations	1,167,712	273,843
Payables to customers	23,229	46,525
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash and cash equivalents	820,436	981,580
Cash restricted or segregated under regulations and other	35,024	56,662
Securities borrowed	0	0
Securities purchased under agreements to resell	0	0
Receivables from broker-dealers and clearing organizations	0	0
Receivables from customers	0	0
Other assets	10,444	0
Total Assets	865,904	1,038,242
Liabilities
Short-term borrowings	0	0
Long-term borrowings	0	0
Securities loaned	0	0
Securities sold under agreements to repurchase	0	0
Payables to broker-dealers and clearing organizations	0	0
Payables to customers	0	0
Other liabilities	0	0
Total Liabilities	0	0
Significant Other Observable Inputs (Level 2)
Assets
Cash and cash equivalents	0	0
Cash restricted or segregated under regulations and other	0	0
Securities borrowed	1,722,440	1,187,674
Securities purchased under agreements to resell	1,512,114	336,999
Receivables from broker-dealers and clearing organizations	737,724	1,027,917
Receivables from customers	106,245	80,830
Other assets	20,578	30,579
Total Assets	4,099,101	2,663,999
Liabilities
Short-term borrowings	0	3,944
Long-term borrowings	1,758,292	1,783,943
Securities loaned	1,329,446	1,060,432
Securities sold under agreements to repurchase	1,795,994	627,549
Payables to broker-dealers and clearing organizations	1,160,051	273,843
Payables to customers	23,229	46,525
Other liabilities	19,300	23,776
Total Liabilities	6,086,312	3,820,012
Significant Unobservable Inputs (Level 3)
Assets
Cash and cash equivalents	0	0
Cash restricted or segregated under regulations and other	0	0
Securities borrowed	0	0
Securities purchased under agreements to resell	0	0
Receivables from broker-dealers and clearing organizations	0	0
Receivables from customers	0	0
Other assets	0	0
Total Assets	0	0
Liabilities
Short-term borrowings	0	0
Long-term borrowings	0	0
Securities loaned	0	0
Securities sold under agreements to repurchase	0	0
Payables to broker-dealers and clearing organizations	0	0
Payables to customers	0	0
Other liabilities	0	0
Total Liabilities	0	0
Carrying Value
Assets
Cash and cash equivalents	820,436	981,580
Cash restricted or segregated under regulations and other	35,024	56,662
Securities borrowed	1,722,440	1,187,674
Securities purchased under agreements to resell	1,512,114	336,999
Receivables from broker-dealers and clearing organizations	737,724	1,027,917
Receivables from customers	106,245	80,830
Other assets	31,022	30,579
Total Assets	4,965,005	3,702,241
Liabilities
Short-term borrowings	0	3,944
Long-term borrowings	1,727,205	1,795,952
Securities loaned	1,329,446	1,060,432
Securities sold under agreements to repurchase	1,795,994	627,549
Payables to broker-dealers and clearing organizations	1,160,051	273,843
Payables to customers	23,229	46,525
Other liabilities	19,300	23,776
Total Liabilities	6,055,225	3,832,021
Fair Value
Assets
Cash and cash equivalents	820,436	981,580
Cash restricted or segregated under regulations and other	35,024	56,662
Securities borrowed	1,722,440	1,187,674
Securities purchased under agreements to resell	1,512,114	336,999
Receivables from broker-dealers and clearing organizations	737,724	1,027,917
Receivables from customers	106,245	80,830
Other assets	31,022	30,579
Total Assets	4,965,005	3,702,241
Liabilities
Short-term borrowings	0	3,944
Long-term borrowings	1,758,292	1,783,943
Securities loaned	1,329,446	1,060,432
Securities sold under agreements to repurchase	1,795,994	627,549
Payables to broker-dealers and clearing organizations	1,160,051	273,843
Payables to customers	23,229	46,525
Other liabilities	19,300	23,776
Total Liabilities	$ 6,086,312	$ 3,820,012