Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - Fair Value, Nonrecurring - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	$ 36,578	$ 213,759
Restricted cash	128,280	127,765
Deposits with clearing organization	54,256	37,885
Securities borrowed	72,267	99,752
Receivables from brokers	69,653	39,716
Securities failed to deliver	9,801	9,212
Clearing organizations	22,287	19,518
Other	3,198	1,693
Total Receivable from brokers, dealers and clearing organizations	177,206	169,891
Receivable from customers	1,178,713	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	57,205	53,983
Investments	75,437	99,169
Drafts payable	9,644
Bank call loans	53,600	69,500
Securities loaned	307,383	244,223
Payable to brokers	199	2,077
Securities failed to receive	22,359	6,457
Other	163,356	169,013
Total payables to brokers, dealers and clearing organizations	493,297	421,770
Payables to customers	381,112	456,958
Securities sold under agreements to repurchase	284,032	277,322
Senior secured notes	124,423	131,094
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	36,578	213,759
Restricted cash	128,280	127,765
Deposits with clearing organization	54,256	37,885
Drafts payable	9,644
Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities borrowed	72,267	99,752
Receivables from brokers	69,653	39,716
Securities failed to deliver	9,801	9,212
Clearing organizations	22,287	19,518
Other	3,198	1,693
Total Receivable from brokers, dealers and clearing organizations	177,206	169,891
Receivable from customers	1,178,713	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	57,205	53,983
Investments	75,437	99,169
Bank call loans	53,600	69,500
Securities loaned	307,383	244,223
Payable to brokers	199	2,077
Securities failed to receive	22,359	6,457
Other	163,356	169,013
Total payables to brokers, dealers and clearing organizations	493,297	421,770
Payables to customers	381,112	456,958
Securities sold under agreements to repurchase	284,032	277,322
Senior secured notes	124,423	131,094
Carrying Value
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	36,578	213,759
Restricted cash	128,280	127,765
Deposits with clearing organization	54,256	37,885
Securities borrowed	72,267	99,752
Receivables from brokers	69,653	39,716
Securities failed to deliver	9,801	9,212
Clearing organizations	22,287	19,518
Other	3,198	1,693
Total Receivable from brokers, dealers and clearing organizations	177,206	169,891
Receivable from customers	1,178,713	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	57,205	53,983
Investments	75,437	99,169
Drafts payable	9,644
Bank call loans	53,600	69,500
Securities loaned	307,383	244,223
Payable to brokers	199	2,077
Securities failed to receive	22,359	6,457
Other	163,356	169,013
Total payables to brokers, dealers and clearing organizations	493,297	421,770
Payables to customers	381,112	456,958
Securities sold under agreements to repurchase	284,032	277,322
Senior secured notes	$ 125,000	$ 125,000