Fair value measurements - Assets and Liabilities Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	$ 45,577	$ 52,768
Mortgage Backed Securities, Other
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	143
Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	23,518	36,384
Level 1 | Mortgage Backed Securities, Other
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	0
Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	22,059	16,384
Level 2 | Mortgage Backed Securities, Other
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	143
Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	0	0
Level 3 | Mortgage Backed Securities, Other
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Securities Sold	0
Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	30,773	112,433
Restricted cash	25,949	25,534
Deposits with clearing organizations	53,957	52,754
Securities borrowed	142,544	127,817
Receivables from brokers	45,021	49,125
Securities failed to deliver	20,203	9,099
Clearing organizations and other	51,599	20,035
Total Receivable from brokers, dealers and clearing organizations	259,367	206,076
Receivable from customers	1,087,314	1,202,764
Notes receivable, net	61,270	57,495
Investments	85,606	79,322
Company-owned life insurance policies	84,000	77,000
Drafts payable	14,521
Bank call loans	56,200
Securities loaned	292,860	320,843
Payable to brokers	1,337	123
Securities failed to receive	29,031	62,646
Clearing organization and other	25,243	166,350
Total payables to brokers, dealers and clearing organizations	348,471	549,962
Payables to customers	363,839	456,475
Securities sold under agreements to repurchase	680,064	161,009
Senior secured notes	108,084	113,233
Fair Value, Nonrecurring | Level 1
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	30,773	112,433
Restricted cash	25,949	25,534
Deposits with clearing organizations	53,957	52,754
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
Investments	0	0
Drafts payable	14,521
Bank call loans	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
Senior secured notes	0	0
Fair Value, Nonrecurring | Level 2
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	0	0
Restricted cash	0	0
Deposits with clearing organizations	0	0
Securities borrowed	142,544	127,817
Receivables from brokers	45,021	49,125
Securities failed to deliver	20,203	9,099
Clearing organizations and other	51,599	20,035
Total Receivable from brokers, dealers and clearing organizations	259,367	206,076
Receivable from customers	1,087,314	1,202,764
Notes receivable, net	61,270	57,495
Investments	85,606	79,322
Drafts payable	0
Bank call loans	56,200
Securities loaned	292,860	320,843
Payable to brokers	1,337	123
Securities failed to receive	29,031	62,646
Clearing organization and other	25,243	166,350
Total payables to brokers, dealers and clearing organizations	348,471	549,962
Payables to customers	363,839	456,475
Securities sold under agreements to repurchase	680,064	161,009
Senior secured notes	108,084	113,233
Fair Value, Nonrecurring | Level 3
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	0	0
Restricted cash	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
Investments	0	0
Drafts payable	0
Bank call loans	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
Senior secured notes	0	0
Carrying Value | Fair Value, Nonrecurring
Financial Instruments, Estimate Not Practicable, Fair Value [Line Items]
Cash and cash equivalents	30,773	112,433
Restricted cash	25,949	25,534
Deposits with clearing organizations	53,957	52,754
Securities borrowed	142,544	127,817
Receivables from brokers	45,021	49,125
Securities failed to deliver	20,203	9,099
Clearing organizations and other	51,599	20,035
Total Receivable from brokers, dealers and clearing organizations	259,367	206,076
Receivable from customers	1,087,314	1,202,764
Notes receivable, net	61,270	57,495
Investments	85,606	79,322
Drafts payable	14,521
Bank call loans	56,200
Securities loaned	292,860	320,843
Payable to brokers	1,337	123
Securities failed to receive	29,031	62,646
Clearing organization and other	25,243	166,350
Total payables to brokers, dealers and clearing organizations	348,471	549,962
Payables to customers	363,839	456,475
Securities sold under agreements to repurchase	680,064	161,009
Senior secured notes	$ 113,050	$ 114,050