Fair value measurements - Schedule of Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Company-owned life insurance	$ 109,094	$ 98,828
Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	38,405	33,150
Deposits with clearing organizations	79,611	70,838
Securities borrowed	160,006	137,177
Receivables from brokers	51,080	59,487
Securities failed to deliver	2,583	8,459
Clearing organizations and other	46,298	36,355
Total Receivable from brokers, dealers and clearing organizations	259,967	241,478
Receivables from customers	1,415,049	1,268,866
Notes receivable, net	57,965	67,931
Company-owned life insurance	109,094	98,828
Investments	2,114	1,634
Drafts payable	18,347	21,661
Bank call loans	76,800	252,100
Securities loaned	370,331	235,498
Payables to brokers	728	607
Securities failed to receive	18,937	14,757
Clearing organizations and other	7,841	1,883
Total payables to brokers, dealers and clearing organizations	397,837	252,745
Payables to customers	393,694	357,835
Securities sold under agreements to repurchase	997,192	931,754
Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	38,405	33,150
Deposits with clearing organizations	79,611	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
Receivables from customers	0	0
Notes receivable, net	0	0
Company-owned life insurance	0	0
Investments	0	0
Drafts payable	18,347	21,661
Bank call loans	0	0
Securities loaned	0	0
Payables to brokers	0	0
Securities failed to receive	0	0
Clearing organizations 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
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	160,006	137,177
Receivables from brokers	51,080	59,487
Securities failed to deliver	2,583	8,459
Clearing organizations and other	46,298	36,355
Total Receivable from brokers, dealers and clearing organizations	259,967	241,478
Receivables from customers	1,415,049	1,268,866
Notes receivable, net	57,965	67,931
Company-owned life insurance	109,094	98,828
Investments	2,114	1,634
Drafts payable	0	0
Bank call loans	76,800	252,100
Securities loaned	370,331	235,498
Payables to brokers	728	607
Securities failed to receive	18,937	14,757
Clearing organizations and other	7,841	1,883
Total payables to brokers, dealers and clearing organizations	397,837	252,745
Payables to customers	393,694	357,835
Securities sold under agreements to repurchase	997,192	931,754
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
Receivables from customers	0	0
Notes receivable, net	0	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 organizations 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 | Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	38,405	33,150
Deposits with clearing organizations	79,611	70,838
Securities borrowed	160,006	137,177
Receivables from brokers	51,080	59,487
Securities failed to deliver	2,583	8,459
Clearing organizations and other	46,298	36,355
Total Receivable from brokers, dealers and clearing organizations	259,967	241,478
Receivables from customers	1,415,049	1,268,866
Notes receivable, net	57,965	67,931
Company-owned life insurance	109,094	98,828
Investments	2,114	1,634
Drafts payable	18,347	21,661
Bank call loans	76,800	252,100
Securities loaned	370,331	235,498
Payables to brokers	728	607
Securities failed to receive	18,937	14,757
Clearing organizations and other	7,841	1,883
Total payables to brokers, dealers and clearing organizations	397,837	252,745
Payables to customers	393,694	357,835
Securities sold under agreements to repurchase	$ 997,192	$ 931,754