Fair value measurements - Schedule of Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Corporate-owned life insurance	$ 107,686	$ 98,828
Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	38,288	33,150
Deposits with clearing organizations	72,015	70,838
Securities borrowed	164,230	137,177
Receivables from brokers	54,255	59,487
Securities failed to deliver	23,975	8,459
Clearing organizations and other	82,799	36,355
Total receivable from brokers, dealers and clearing organizations	325,259	241,478
Receivable from customers	1,374,526	1,268,866
Notes receivable, net	67,366	67,931
Corporate-owned life insurance	107,686	98,828
Investments	2,205	1,634
Drafts payable	16,763	21,661
Bank call loans	262,300	252,100
Securities loaned	286,694	235,498
Payable to brokers	1,379	607
Securities failed to receive	29,530	14,757
Clearing organization and other	3,668	1,883
Total payables to brokers, dealers and clearing organizations	321,271	252,745
Payables to customers	465,426	357,835
Securities sold under agreements to repurchase	972,167	931,754
Fair Value, Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	38,288	33,150
Deposits with clearing organizations	72,015	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	16,763	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	164,230	137,177
Receivables from brokers	54,255	59,487
Securities failed to deliver	23,975	8,459
Clearing organizations and other	82,799	36,355
Total receivable from brokers, dealers and clearing organizations	325,259	241,478
Receivable from customers	1,374,526	1,268,866
Notes receivable, net	67,366	67,931
Corporate-owned life insurance	107,686	98,828
Investments	2,205	1,634
Drafts payable	0	0
Bank call loans	262,300	252,100
Securities loaned	286,694	235,498
Payable to brokers	1,379	607
Securities failed to receive	29,530	14,757
Clearing organization and other	3,668	1,883
Total payables to brokers, dealers and clearing organizations	321,271	252,745
Payables to customers	465,426	357,835
Securities sold under agreements to repurchase	972,167	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	38,288	33,150
Deposits with clearing organizations	72,015	70,838
Securities borrowed	164,230	137,177
Receivables from brokers	54,255	59,487
Securities failed to deliver	23,975	8,459
Clearing organizations and other	82,799	36,355
Total receivable from brokers, dealers and clearing organizations	325,259	241,478
Receivable from customers	1,374,526	1,268,866
Notes receivable, net	67,366	67,931
Corporate-owned life insurance	107,686	98,828
Investments	2,205	1,634
Drafts payable	16,763	21,661
Bank call loans	262,300	252,100
Securities loaned	286,694	235,498
Payable to brokers	1,379	607
Securities failed to receive	29,530	14,757
Clearing organization and other	3,668	1,883
Total payables to brokers, dealers and clearing organizations	321,271	252,745
Payables to customers	465,426	357,835
Securities sold under agreements to repurchase	$ 972,167	$ 931,754