Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - Fair Value, Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	$ 29,145	$ 112,433
Restricted cash	25,803	25,534
Deposits with clearing organization	48,619	52,754
Securities borrowed	255,568	127,817
Receivables from brokers	48,354	49,125
Securities failed to deliver	5,509	9,099
Clearing organizations and other	24,751	20,035
Total Receivable from brokers, dealers and clearing organizations	334,182	206,076
Receivable from customers	1,066,439	1,202,764
Securities purchased under agreements to resell		0
Notes receivable, net	60,873	57,495
Investments	87,826	79,322
Company-owned life insurance policies	86,000	77,000
Drafts payable	21,734
Bank call loans	94,400	0
Securities loaned	336,513	320,843
Payable to brokers	553	123
Securities failed to receive	5,656	62,646
Clearing organization and other	75,624	166,350
Total payables to brokers, dealers and clearing organizations	418,346	549,962
Payables to customers	401,168	456,475
Securities sold under agreements to repurchase	645,315	161,009
Senior secured notes	107,702	113,233
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	29,145	112,433
Restricted cash	25,803	25,534
Deposits with clearing organization	48,619	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
Securities purchased under agreements to resell		0
Notes receivable, net	0	0
Investments	0	0
Drafts payable	21,734
Bank call loans	0	0
Securities loaned	0	0
Payable to brokers	0	0
Securities failed to receive	0	0
Clearing organization 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
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	0	0
Restricted cash	0	0
Deposits with clearing organization	0	0
Securities borrowed	255,568	127,817
Receivables from brokers	48,354	49,125
Securities failed to deliver	5,509	9,099
Clearing organizations and other	24,751	20,035
Total Receivable from brokers, dealers and clearing organizations	334,182	206,076
Receivable from customers	1,066,439	1,202,764
Securities purchased under agreements to resell		0
Notes receivable, net	60,873	57,495
Investments	87,826	79,322
Drafts payable	0
Bank call loans	94,400	0
Securities loaned	336,513	320,843
Payable to brokers	553	123
Securities failed to receive	5,656	62,646
Clearing organization and other	75,624	166,350
Total payables to brokers, dealers and clearing organizations	418,346	549,962
Payables to customers	401,168	456,475
Securities sold under agreements to repurchase	645,315	161,009
Senior secured notes	107,702	113,233
Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	0	0
Restricted cash	0	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
Securities purchased under agreements to resell		0
Notes receivable, net	0	0
Investments	0	0
Drafts payable	0
Bank call loans	0	0
Securities loaned	0	0
Payable to brokers	0	0
Securities failed to receive	0	0
Clearing organization 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
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	29,145	112,433
Restricted cash	25,803	25,534
Deposits with clearing organization	48,619	52,754
Securities borrowed	255,568	127,817
Receivables from brokers	48,354	49,125
Securities failed to deliver	5,509	9,099
Clearing organizations and other	24,751	20,035
Total Receivable from brokers, dealers and clearing organizations	334,182	206,076
Receivable from customers	1,066,439	1,202,764
Securities purchased under agreements to resell		0
Notes receivable, net	60,873	57,495
Investments	87,826	79,322
Drafts payable	21,734
Bank call loans	94,400	0
Securities loaned	336,513	320,843
Payable to brokers	553	123
Securities failed to receive	5,656	62,646
Clearing organization and other	75,624	166,350
Total payables to brokers, dealers and clearing organizations	418,346	549,962
Payables to customers	401,168	456,475
Securities sold under agreements to repurchase	645,315	161,009
Senior secured notes	$ 113,050	$ 114,050