Fair value measurements - Schedule of Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell	$ 4,079	$ 5,842
Corporate-owned life insurance	94,646	88,989
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell		0
Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell		5,842
Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities purchased under agreements to resell		0
Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	33,214	28,835
Deposits with clearing organizations	73,448	43,917
Securities borrowed	134,454	158,612
Receivables from brokers	57,699	65,639
Securities failed to deliver	20,054	29,656
Clearing organizations and other	84,828	30,780
Total receivable from brokers, dealers and clearing organizations	297,035	284,687
Receivable from customers	1,161,685	1,059,892
Notes receivable, net	63,342	62,640
Securities purchased under agreements to resell	4,079
Corporate-owned life insurance	94,646	88,989
Investments	1,647	2,010
Drafts payable	21,762	9,002
Bank call loans	218,800
Securities loaned	247,219	284,987
Payable to brokers	1,557	447
Securities failed to receive	22,961	23,809
Clearing organization and other	5,066	51,912
Total payables to brokers, dealers and clearing organizations	276,803	361,155
Payables to customers	325,886	369,287
Securities sold under agreements to repurchase	822,785	640,382
Senior secured notes	111,637	109,838
Fair Value, Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	33,214	28,835
Deposits with clearing organizations	73,448	43,917
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
Securities purchased under agreements to resell	0
Corporate-owned life insurance	0	0
Investments	0	0
Drafts payable	21,762	9,002
Bank call loans	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
Fair Value, Nonrecurring | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	0	0
Deposits with clearing organizations	0	0
Securities borrowed	134,454	158,612
Receivables from brokers	57,699	65,639
Securities failed to deliver	20,054	29,656
Clearing organizations and other	84,828	30,780
Total receivable from brokers, dealers and clearing organizations	297,035	284,687
Receivable from customers	1,161,685	1,059,892
Notes receivable, net	63,342	62,640
Securities purchased under agreements to resell	4,079
Corporate-owned life insurance	94,646	88,989
Investments	1,647	2,010
Drafts payable	0	0
Bank call loans	218,800
Securities loaned	247,219	284,987
Payable to brokers	1,557	447
Securities failed to receive	22,961	23,809
Clearing organization and other	5,066	51,912
Total payables to brokers, dealers and clearing organizations	276,803	361,155
Payables to customers	325,886	369,287
Securities sold under agreements to repurchase	822,785	640,382
Senior secured notes	111,637	109,838
Fair Value, Nonrecurring | Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	0	0
Deposits with clearing organizations	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
Securities purchased under agreements to resell	0
Corporate-owned life insurance	0	0
Investments	0	0
Drafts payable	0	0
Bank call loans	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 | Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	33,214	28,835
Deposits with clearing organizations	73,448	43,917
Securities borrowed	134,454	158,612
Receivables from brokers	57,699	65,639
Securities failed to deliver	20,054	29,656
Clearing organizations and other	84,828	30,780
Total receivable from brokers, dealers and clearing organizations	297,035	284,687
Receivable from customers	1,161,685	1,059,892
Notes receivable, net	63,342	62,640
Securities purchased under agreements to resell	4,079
Corporate-owned life insurance	94,646	88,989
Investments	1,647	2,010
Drafts payable	21,762	9,002
Bank call loans	218,800
Securities loaned	247,219	284,987
Payable to brokers	1,557	447
Securities failed to receive	22,961	23,809
Clearing organization and other	5,066	51,912
Total payables to brokers, dealers and clearing organizations	276,803	361,155
Payables to customers	325,886	369,287
Securities sold under agreements to repurchase	822,785	640,382
Senior secured notes	$ 113,050	$ 113,050