Financial Assets and Liabilities - Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Assets
Cash restricted or segregated under regulations and other	$ 31,857	$ 35,024
Receivables from broker-dealers and clearing organizations	902,867	737,724
Receivables from customers	124,769	106,245
Other assets	341,949	303,720
Liabilities
Payables to broker-dealers and clearing organizations	843,112	1,167,712
Payables to customers	58,708	23,229
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash and cash equivalents	684,806	820,436
Cash restricted or segregated under regulations and other	31,857	35,024
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	10,662	10,444
Total Assets	727,325	865,904
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	1,918,973	1,722,440
Securities purchased under agreements to resell	793,292	1,512,114
Receivables from broker-dealers and clearing organizations	875,185	737,724
Receivables from customers	124,769	106,245
Other assets	22,432	20,578
Total Assets	3,734,651	4,099,101
Liabilities
Short-term borrowings	75,000	0
Long-term borrowings	1,768,972	1,758,292
Securities loaned	1,557,661	1,329,446
Securities sold under agreements to repurchase	1,072,043	1,795,994
Payables to broker-dealers and clearing organizations	799,344	1,160,051
Payables to customers	58,708	23,229
Other liabilities	26,262	19,300
Total Liabilities	5,357,990	6,086,312
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	684,806	820,436
Cash restricted or segregated under regulations and other	31,857	35,024
Securities borrowed	1,918,973	1,722,440
Securities purchased under agreements to resell	793,292	1,512,114
Receivables from broker-dealers and clearing organizations	875,185	737,724
Receivables from customers	124,769	106,245
Other assets	33,094	31,022
Total Assets	4,461,976	4,965,005
Liabilities
Short-term borrowings	73,692	0
Long-term borrowings	1,738,056	1,727,205
Securities loaned	1,557,661	1,329,446
Securities sold under agreements to repurchase	1,072,043	1,795,994
Payables to broker-dealers and clearing organizations	799,344	1,160,051
Payables to customers	58,708	23,229
Other liabilities	26,262	19,300
Total Liabilities	5,325,766	6,055,225
Fair Value
Assets
Cash and cash equivalents	684,806	820,436
Cash restricted or segregated under regulations and other	31,857	35,024
Securities borrowed	1,918,973	1,722,440
Securities purchased under agreements to resell	793,292	1,512,114
Receivables from broker-dealers and clearing organizations	875,185	737,724
Receivables from customers	124,769	106,245
Other assets	33,094	31,022
Total Assets	4,461,976	4,965,005
Liabilities
Short-term borrowings	75,000	0
Long-term borrowings	1,768,972	1,758,292
Securities loaned	1,557,661	1,329,446
Securities sold under agreements to repurchase	1,072,043	1,795,994
Payables to broker-dealers and clearing organizations	799,344	1,160,051
Payables to customers	58,708	23,229
Other liabilities	26,262	19,300
Total Liabilities	$ 5,357,990	$ 6,086,312