Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 28,835	$ 112,433
Restricted cash		25,534
Deposits with clearing organization	43,917	52,754
Securities borrowed	158,612	127,817
Receivables from brokers	65,639	49,125
Securities failed to deliver	29,656	9,099
Clearing organizations and Other	30,780	20,035
Total Receivable from brokers, dealers and clearing organizations	284,687	206,076
Receivable from customers	1,059,892	1,202,764
Notes receivable, net	62,640	57,495
Investments	90,999	79,322
Drafts payable	9,002
Securities loaned	284,987	320,843
Payable to brokers	447	123
Securities failed to receive	23,809	62,646
Clearing Organizations and Other	51,912	166,350
Total payables to brokers, dealers and clearing organizations	361,155	549,962
Payables to customers	369,287	456,475
Securities sold under agreements to repurchase	640,382	161,009
Senior secured notes	109,838	113,233
Underlying investment	89,000	77,000
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	28,835	112,433
Restricted cash		25,534
Deposits with clearing organization	43,917	52,754
Securities borrowed	0	0
Receivables from brokers	0	0
Securities failed to deliver	0	0
Clearing organizations and Other	0	0
Total Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers	0	0
Notes receivable, net	0	0
Investments	0	0
Drafts payable	9,002
Securities loaned	0	0
Payable to brokers	0	0
Securities failed to receive	0	0
Clearing Organizations and Other	0	0
Total payables to brokers, dealers and clearing organizations	0	0
Payables to customers	0	0
Securities sold under agreements to repurchase	0	0
Senior secured notes	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Restricted cash		0
Deposits with clearing organization	0	0
Securities borrowed	158,612	127,817
Receivables from brokers	65,639	49,125
Securities failed to deliver	29,656	9,099
Clearing organizations and Other	30,780	20,035
Total Receivable from brokers, dealers and clearing organizations	284,687	206,076
Receivable from customers	1,059,892	1,202,764
Notes receivable, net	62,640	57,495
Investments	90,999	79,322
Drafts payable	0
Securities loaned	284,987	320,843
Payable to brokers	447	123
Securities failed to receive	23,809	62,646
Clearing Organizations and Other	51,912	166,350
Total payables to brokers, dealers and clearing organizations	361,155	549,962
Payables to customers	369,287	456,475
Securities sold under agreements to repurchase	640,382	161,009
Senior secured notes	109,838	113,233
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Restricted cash		0
Deposits with clearing organization	0	0
Securities borrowed	0	0
Receivables from brokers	0	0
Securities failed to deliver	0	0
Clearing organizations and Other	0	0
Total Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers	0	0
Notes receivable, net	0	0
Investments	0	0
Drafts payable	0
Securities loaned	0	0
Payable to brokers	0	0
Securities failed to receive	0	0
Clearing Organizations and Other	0	0
Total payables to brokers, dealers and clearing organizations	0	0
Payables to customers	0	0
Securities sold under agreements to repurchase	0	0
Senior secured notes	0	0
Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	28,835	112,433
Restricted cash		25,534
Deposits with clearing organization	43,917	52,754
Securities borrowed	158,612	127,817
Receivables from brokers	65,639	49,125
Securities failed to deliver	29,656	9,099
Clearing organizations and Other	30,780	20,035
Total Receivable from brokers, dealers and clearing organizations	284,687	206,076
Receivable from customers	1,059,892	1,202,764
Notes receivable, net	62,640	57,495
Investments	90,999	79,322
Drafts payable	9,002
Securities loaned	284,987	320,843
Payable to brokers	447	123
Securities failed to receive	23,809	62,646
Clearing Organizations and Other	51,912	166,350
Total payables to brokers, dealers and clearing organizations	361,155	549,962
Payables to customers	369,287	456,475
Securities sold under agreements to repurchase	640,382	161,009
Senior secured notes	$ 113,050	$ 114,050