Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Corporate-owned life insurance	$ 98,828	$ 88,989
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	33,150	28,835
Deposits with clearing organizations	70,838	43,917
Securities borrowed	137,177	158,612
Receivables from brokers	59,487	65,639
Securities failed to deliver	8,459	29,656
Clearing organizations and other	36,355	30,780
Total Receivable from brokers, dealers and clearing organizations	241,478	284,687
Receivable from customers	1,268,866	1,059,892
Notes receivable, net	67,931	62,640
Corporate-owned life insurance	98,828	88,989
Investments	1,634	2,010
Drafts payable	21,661	9,002
Bank call loans	252,100
Securities loaned	235,498	284,987
Payable to brokers	607	447
Securities failed to receive	14,757	23,809
Clearing Organizations and Other	1,883	51,912
Total payables to brokers, dealers and clearing organizations	252,745	361,155
Payables to customers	357,835	369,287
Securities sold under agreements to repurchase	931,754	640,382
Senior secured notes		109,838
Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	33,150	28,835
Deposits with clearing organizations	70,838	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
Corporate-owned life insurance
Investments	0	0
Drafts payable	21,661	9,002
Bank call loans	0
Securities loaned	0	0
Payable 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
Senior secured notes		0
Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Deposits with clearing organizations	0	0
Securities borrowed	137,177	158,612
Receivables from brokers	59,487	65,639
Securities failed to deliver	8,459	29,656
Clearing organizations and other	36,355	30,780
Total Receivable from brokers, dealers and clearing organizations	241,478	284,687
Receivable from customers	1,268,866	1,059,892
Notes receivable, net	67,931	62,640
Corporate-owned life insurance	98,828	88,989
Investments	1,634	2,010
Drafts payable	0	0
Bank call loans	252,100
Securities loaned	235,498	284,987
Payable to brokers	607	447
Securities failed to receive	14,757	23,809
Clearing Organizations and Other	1,883	51,912
Total payables to brokers, dealers and clearing organizations	252,745	361,155
Payables to customers	357,835	369,287
Securities sold under agreements to repurchase	931,754	640,382
Senior secured notes		109,838
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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
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 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
Senior secured notes		0
Nonrecurring | Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	33,150	28,835
Deposits with clearing organizations	70,838	43,917
Securities borrowed	137,177	158,612
Receivables from brokers	59,487	65,639
Securities failed to deliver	8,459	29,656
Clearing organizations and other	36,355	30,780
Total Receivable from brokers, dealers and clearing organizations	241,478	284,687
Receivable from customers	1,268,866	1,059,892
Notes receivable, net	67,931	62,640
Corporate-owned life insurance	98,828	88,989
Investments	1,634	2,010
Drafts payable	21,661	9,002
Bank call loans	252,100
Securities loaned	235,498	284,987
Payable to brokers	607	447
Securities failed to receive	14,757	23,809
Clearing Organizations and Other	1,883	51,912
Total payables to brokers, dealers and clearing organizations	252,745	361,155
Payables to customers	357,835	369,287
Securities sold under agreements to repurchase	$ 931,754	640,382
Senior secured notes		$ 113,050