Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - Fair Value, Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	$ 36,606	$ 213,759
Restricted cash	127,875	127,765
Deposits with clearing organization	58,379	37,885
Securities borrowed	79,038	99,752
Receivables from brokers	42,255	39,716
Securities failed to deliver	26,705	9,212
Clearing organizations	19,355	19,518
Other	548	1,693
Total Receivable from brokers, dealers and clearing organizations	167,901	169,891
Receivable from customers	1,288,079	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	59,099	53,983
Investments	78,246	99,169
Drafts payable	10,020
Bank call loans	177,300	69,500
Securities loaned	281,407	244,223
Payable to brokers	3,357	2,077
Securities failed to receive	50,028	6,457
Other	53,304	169,013
Total payables to brokers, dealers and clearing organizations	388,096	421,770
Payables to customers	419,315	456,958
Securities sold under agreements to repurchase	170,968	277,322
Senior secured notes	125,213	131,094
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	36,606	213,759
Restricted cash	127,875	127,765
Deposits with clearing organization	58,379	37,885
Drafts payable	10,020
Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities borrowed	79,038	99,752
Receivables from brokers	42,255	39,716
Securities failed to deliver	26,705	9,212
Clearing organizations	19,355	19,518
Other	548	1,693
Total Receivable from brokers, dealers and clearing organizations	167,901	169,891
Receivable from customers	1,288,079	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	59,099	53,983
Investments	78,246	99,169
Bank call loans	177,300	69,500
Securities loaned	281,407	244,223
Payable to brokers	3,357	2,077
Securities failed to receive	50,028	6,457
Other	53,304	169,013
Total payables to brokers, dealers and clearing organizations	388,096	421,770
Payables to customers	419,315	456,958
Securities sold under agreements to repurchase	170,968	277,322
Senior secured notes	125,213	131,094
Carrying Value
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash	36,606	213,759
Restricted cash	127,875	127,765
Deposits with clearing organization	58,379	37,885
Securities borrowed	79,038	99,752
Receivables from brokers	42,255	39,716
Securities failed to deliver	26,705	9,212
Clearing organizations	19,355	19,518
Other	548	1,693
Total Receivable from brokers, dealers and clearing organizations	167,901	169,891
Receivable from customers	1,288,079	1,221,450
Securities purchased under agreements to resell		935
Notes receivable, net	59,099	53,983
Investments	78,246	99,169
Drafts payable	10,020
Bank call loans	177,300	69,500
Securities loaned	281,407	244,223
Payable to brokers	3,357	2,077
Securities failed to receive	50,028	6,457
Other	53,304	169,013
Total payables to brokers, dealers and clearing organizations	388,096	421,770
Payables to customers	419,315	456,958
Securities sold under agreements to repurchase	170,968	277,322
Senior secured notes	$ 125,000	$ 125,000