Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable, net	$ 53,983	$ 46,161
Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash	213,759	35,424
Restricted cash	127,765
Deposits with clearing organization	37,885	59,352
Securities borrowed	99,752	110,932
Receivables from brokers	39,716	30,133
Securities failed to deliver	9,212	17,840
Clearing organizations	19,518	28,955
Other	1,693	15,622
Total Receivable from brokers, dealers and clearing organizations	169,891	203,482
Receivable from customers	1,221,450	1,110,835
Securities purchased under agreements to resell	935
Notes receivable, net	53,983	46,161
Investments	99,169	85,552
Bank call loans	69,500	82,000
Securities loaned	244,223	249,499
Payable to brokers	2,077	4,102
Securities failed to receive	6,457	6,218
Other	169,013	70
Total payables to brokers, dealers and clearing organizations	421,770	259,889
Payables to customers	456,958	502,807
Securities sold under agreements to repurchase	277,322	342,438
Senior secured notes	131,094	127,033
Fair Value, Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash	213,759	35,424
Restricted cash	127,765
Deposits with clearing organization	37,885	59,352
Fair Value, Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities borrowed	99,752	110,932
Receivables from brokers	39,716	30,133
Securities failed to deliver	9,212	17,840
Clearing organizations	19,518	28,955
Other	1,693	15,622
Total Receivable from brokers, dealers and clearing organizations	169,891	203,482
Receivable from customers	1,221,450	1,110,835
Securities purchased under agreements to resell	935
Notes receivable, net	53,983	46,161
Investments	99,169	85,552
Bank call loans	69,500	82,000
Securities loaned	244,223	249,499
Payable to brokers	2,077	4,102
Securities failed to receive	6,457	6,218
Other	169,013	70
Total payables to brokers, dealers and clearing organizations	421,770	259,889
Payables to customers	456,958	502,807
Securities sold under agreements to repurchase	277,322	342,438
Senior secured notes	131,094	127,033
Fair Value, Nonrecurring | Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash	213,759	35,424
Restricted cash	127,765
Deposits with clearing organization	37,885	59,352
Securities borrowed	99,752	110,932
Receivables from brokers	39,716	30,133
Securities failed to deliver	9,212	17,840
Clearing organizations	19,518	28,955
Other	1,693	15,622
Total Receivable from brokers, dealers and clearing organizations	169,891	203,482
Receivable from customers	1,221,450	1,110,835
Securities purchased under agreements to resell	935
Notes receivable, net	53,983	46,161
Investments	99,169	85,552
Bank call loans	69,500	82,000
Securities loaned	244,223	249,499
Payable to brokers	2,077	4,102
Securities failed to receive	6,457	6,218
Other	169,013	70
Total payables to brokers, dealers and clearing organizations	421,770	259,889
Payables to customers	456,958	502,807
Securities sold under agreements to repurchase	277,322	342,438
Senior secured notes	$ 125,000	$ 125,000