Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Cash Flows [Abstract]
Net income	$ 158,964,000	$ 122,986,000	$ 52,953,000
Non-cash items included in net income:
Depreciation and amortization of furniture, equipment and leasehold improvements	7,994,000	8,143,000	7,635,000
Deferred income taxes	(792,000)	21,336,000	9,878,000
Amortization of notes receivable	13,427,000	11,763,000	12,971,000
Amortization of debt issuance costs	250,000	206,000	236,000
Write-off of debt issuance costs	0	341,000	184,000
Provision for (reversal of) credit losses	2,896,000	(41,000)	(435,000)
Share-based compensation	31,138,000	16,220,000	11,819,000
Amortization of right-of-use lease assets	26,125,000	25,150,000	25,186,000
Gain on repurchase of senior secured notes	0	(86,000)	0
Decrease (increase) in operating assets:
Deposits with clearing organizations	16,375,000	(34,928,000)	19,263,000
Receivable from brokers, dealers and clearing organizations	33,592,000	(40,201,000)	3,200,000
Receivable from customers	(113,511,000)	(313,860,000)	(75,722,000)
Income tax receivable	0	5,170,000	(4,156,000)
Securities purchased under agreements to resell	(935,000)	0	290,000
Securities owned	(23,987,000)	189,202,000	37,865,000
Notes receivable	(21,249,000)	(14,254,000)	(12,583,000)
Other assets	65,814,000	(105,435,000)	(53,698,000)
Increase (decrease) in operating liabilities:
Drafts payable	0	0	(16,348,000)
Payable to brokers, dealers and clearing organizations	162,146,000	(261,064,000)	231,768,000
Payable to customers	(45,849,000)	168,072,000	(1,881,000)
Securities sold under agreements to repurchase	(65,116,000)	55,173,000	(196,953,000)
Securities sold but not yet purchased	(54,213,000)	25,600,000	15,125,000
Accrued compensation	23,238,000	82,367,000	36,319,000
Income tax payable	3,810,000	9,726,000	0
Accounts payable and other liabilities	7,669,000	(25,645,000)	(23,774,000)
Cash provided by/(used in) operating activities	227,786,000	(54,059,000)	79,142,000
Cash flows from investing activities
Purchase of furniture, equipment and leasehold improvements	(8,268,000)	(4,528,000)	(10,024,000)
Proceeds from the settlement of Company-owned life insurance	2,001,000	587,000	1,720,000
Cash used in investing activities	(6,267,000)	(3,941,000)	(8,304,000)
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	(19,444,000)	(18,578,000)	(5,944,000)
Issuance of Class A non-voting common stock	58,000	56,000	83,000
Repurchase of Class A non-voting common stock for cancellation	(7,737,000)	(15,048,000)	(8,400,000)
Payments for employee taxes withheld related to vested share-based awards	(4,967,000)	(5,839,000)	(1,014,000)
Issuance of senior secured notes	0	125,000,000	0
Payment of Company sponsored Initial Public Offering costs	(454,000)	0	0
Contributions from noncontrolling interests	3,147,000	0	0
Proceeds from Company sponsored Initial Public Offering	126,500,000	0	0
Redemption of senior secured notes	0	(148,574,000)	(50,000,000)
Repurchase of senior secured notes	0	(1,426,000)	0
Debt issuance costs	(22,000)	(1,210,000)	0
Debt redemption costs	0	(2,507,000)	(1,688,000)
(Decrease)/increase in bank call loans, net	(12,500,000)	82,000,000	(15,000,000)
Cash provided by/(used in) financing activities	84,581,000	13,874,000	(81,963,000)
Net increase/(decrease) in cash and cash equivalents and restricted cash	306,100,000	(44,126,000)	(11,125,000)
Cash and cash equivalents and restricted cash, beginning of year	35,424,000	79,550,000	90,675,000
Cash and cash equivalents and restricted cash, end of year	341,524,000	35,424,000	79,550,000
Reconciliation of cash and cash equivalents and restricted cash within the consolidated balance sheet:
Cash and cash equivalents	213,759,000	35,424,000	79,550,000
Restricted cash	127,765,000	0	0
Total cash and cash equivalents and restricted cash	341,524,000	35,424,000	79,550,000
Schedule of non-cash financing activities
Employee share plan issuance	7,361,000	12,167,000	2,192,000
Supplemental disclosure of cash flow information
Cash paid during the year for interest	10,089,000	19,013,000	40,678,000
Cash paid during the year for income taxes, net	$ 62,378,000	$ 11,191,000	$ 16,816,000