Fair value measurements - Schedule of Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Company-owned life insurance	$ 106,144	$ 109,094
Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	34,601	38,405
Deposits with clearing organizations	85,993	79,611
Securities borrowed	158,057	160,006
Receivables from brokers	48,775	51,080
Securities failed to deliver	20,360	2,583
Clearing organizations	28,846	27,215
Trade date receivables	33,430	14,800
Other	5,810	4,283
Total receivable from brokers, dealers and clearing organizations	295,278	259,967
Receivables from customers	1,483,755	1,415,049
Notes receivable, net	55,137	57,965
Company-owned life insurance	106,144	109,094
Investments	2,924	2,114
Drafts payable	19,754	18,347
Bank call loans	287,900	76,800
Securities loaned	320,163	370,331
Payables to brokers	1,136	728
Securities failed to receive	49,557	18,937
Clearing organization and other	173	7,841
Total payables to brokers, dealers and clearing organizations	371,029	397,837
Payables to customers	376,343	393,694
Securities sold under agreements to repurchase	968,346	997,192
Fair Value, Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	34,601	38,405
Deposits with clearing organizations	85,993	79,611
Securities borrowed	0	0
Receivables from brokers	0	0
Securities failed to deliver	0	0
Clearing organizations	0	0
Trade date receivables	0	0
Other	0	0
Total receivable from brokers, dealers and clearing organizations	0	0
Receivables from customers	0	0
Notes receivable, net	0	0
Company-owned life insurance	0	0
Investments	0	0
Drafts payable	19,754	18,347
Bank call loans	0	0
Securities loaned	0	0
Payables 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
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	158,057	160,006
Receivables from brokers	48,775	51,080
Securities failed to deliver	20,360	2,583
Clearing organizations	28,846	27,215
Trade date receivables	33,430	14,800
Other	5,810	4,283
Total receivable from brokers, dealers and clearing organizations	295,278	259,967
Receivables from customers	1,483,755	1,415,049
Notes receivable, net	55,137	57,965
Company-owned life insurance	106,144	109,094
Investments	2,924	2,114
Drafts payable	0	0
Bank call loans	287,900	76,800
Securities loaned	320,163	370,331
Payables to brokers	1,136	728
Securities failed to receive	49,557	18,937
Clearing organization and other	173	7,841
Total payables to brokers, dealers and clearing organizations	371,029	397,837
Payables to customers	376,343	393,694
Securities sold under agreements to repurchase	968,346	997,192
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	0	0
Trade date receivables	0	0
Other	0	0
Total receivable from brokers, dealers and clearing organizations	0	0
Receivables from customers	0	0
Notes receivable, net		0
Company-owned life insurance	0	0
Investments	0	0
Drafts payable	0	0
Bank call loans	0	0
Securities loaned	0	0
Payables 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
Carrying Value | Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	34,601	38,405
Deposits with clearing organizations	85,993	79,611
Securities borrowed	158,057	160,006
Receivables from brokers	48,775	51,080
Securities failed to deliver	20,360	2,583
Clearing organizations	28,846	27,215
Trade date receivables	33,430	14,800
Other	5,810	4,283
Total receivable from brokers, dealers and clearing organizations	295,278	259,967
Receivables from customers	1,483,755	1,415,049
Notes receivable, net	55,137	57,965
Company-owned life insurance	106,144	109,094
Investments	2,924	2,114
Drafts payable	19,754	18,347
Bank call loans	287,900	76,800
Securities loaned	320,163	370,331
Payables to brokers	1,136	728
Securities failed to receive	49,557	18,937
Clearing organization and other	173	7,841
Total payables to brokers, dealers and clearing organizations	371,029	397,837
Payables to customers	376,343	393,694
Securities sold under agreements to repurchase	$ 968,346	$ 997,192