Fair value measurements - Schedule of Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Corporate-owned life insurance	$ 104,045	$ 98,828
Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	37,618	33,150
Deposits with clearing organizations	115,349	70,838
Securities borrowed	139,852	137,177
Receivables from brokers	53,263	59,487
Securities failed to deliver	39,665	8,459
Clearing organizations and other	57,401	36,355
Total receivable from brokers, dealers and clearing organizations	290,181	241,478
Receivable from customers	1,341,550	1,268,866
Notes receivable, net	64,316	67,931
Corporate-owned life insurance	104,045	98,828
Investments	2,016	1,634
Drafts payable	12,128	21,661
Bank call loans	323,800	252,100
Securities loaned	404,916	235,498
Payable to brokers	3,099	607
Securities failed to receive	20,751	14,757
Clearing organization and other	97,353	1,883
Total payables to brokers, dealers and clearing organizations	526,119	252,745
Payables to customers	363,535	357,835
Securities sold under agreements to repurchase	853,123	931,754
Fair Value, Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	37,618	33,150
Deposits with clearing organizations	115,349	70,838
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
Corporate-owned life insurance	0	0
Investments	0	0
Drafts payable	12,128	21,661
Bank call loans	0	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
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	139,852	137,177
Receivables from brokers	53,263	59,487
Securities failed to deliver	39,665	8,459
Clearing organizations and other	57,401	36,355
Total receivable from brokers, dealers and clearing organizations	290,181	241,478
Receivable from customers	1,341,550	1,268,866
Notes receivable, net	64,316	67,931
Corporate-owned life insurance	104,045	98,828
Investments	2,016	1,634
Drafts payable	0	0
Bank call loans	323,800	252,100
Securities loaned	404,916	235,498
Payable to brokers	3,099	607
Securities failed to receive	20,751	14,757
Clearing organization and other	97,353	1,883
Total payables to brokers, dealers and clearing organizations	526,119	252,745
Payables to customers	363,535	357,835
Securities sold under agreements to repurchase	853,123	931,754
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
Corporate-owned life insurance	0	0
Investments	0	0
Drafts payable	0	0
Bank call loans	0	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
Carrying Value | Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	37,618	33,150
Deposits with clearing organizations	115,349	70,838
Securities borrowed	139,852	137,177
Receivables from brokers	53,263	59,487
Securities failed to deliver	39,665	8,459
Clearing organizations and other	57,401	36,355
Total receivable from brokers, dealers and clearing organizations	290,181	241,478
Receivable from customers	1,341,550	1,268,866
Notes receivable, net	64,316	67,931
Corporate-owned life insurance	104,045	98,828
Investments	2,016	1,634
Drafts payable	12,128	21,661
Bank call loans	323,800	252,100
Securities loaned	404,916	235,498
Payable to brokers	3,099	607
Securities failed to receive	20,751	14,757
Clearing organization and other	97,353	1,883
Total payables to brokers, dealers and clearing organizations	526,119	252,745
Payables to customers	363,535	357,835
Securities sold under agreements to repurchase	$ 853,123	$ 931,754