Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Cash Flows [Abstract]
Net income	$ 122,986,000	$ 52,953,000	$ 28,892,000
Non-cash items included in net income:
Depreciation and amortization of furniture, equipment and leasehold improvements	8,143,000	7,635,000	6,871,000
Deferred Income Taxes and Tax Credits	21,336,000	9,878,000	2,773,000
Amortization of notes receivable	11,763,000	12,971,000	12,540,000
Amortization of debt issuance costs	206,000	236,000	259,000
Write-off of debt issuance costs	341,000	184,000	0
Provision for (reversal of) credit losses	(41,000)	(435,000)	117,000
Share-based compensation	16,220,000	11,819,000	6,710,000
Operating Lease Right-of-use Asset, Amortization	25,150,000	25,186,000	0
Gain (Loss) on Repurchase of Debt Instrument	(86,000)
Decrease (increase) in operating assets:
Deposits with clearing organizations	(34,928,000)	19,263,000	(25,456,000)
Receivable from brokers, dealers and clearing organizations	(40,201,000)	3,200,000	20,622,000
Increase (Decrease) in Accounts and Other Receivables	313,860,000	75,722,000	(127,332,000)
Income tax receivable	5,170,000	(4,156,000)	1,925,000
Securities purchased under agreements to resell	0	290,000	368,000
Securities owned	189,202,000	37,865,000	89,013,000
Notes receivable	(14,254,000)	(12,583,000)	(16,078,000)
Other assets	(105,435,000)	(53,698,000)	30,272,000
Increase (decrease) in operating liabilities:
Drafts payable	0	(16,348,000)	(26,064,000)
Payable to brokers, dealers and clearing organizations	(261,064,000)	231,768,000	77,724,000
Payable to customers	168,072,000	(1,881,000)	(49,291,000)
Securities sold under agreements to repurchase	55,173,000	(196,953,000)	(102,260,000)
Securities sold but not yet purchased	25,600,000	15,125,000	(9,040,000)
Accrued compensation	82,367,000	36,319,000	(6,418,000)
Increase (Decrease) in Income Taxes Payable	9,726,000
Accounts payable and other liabilities	(25,645,000)	(23,774,000)	(2,241,000)
Cash (used in) provided by operating activities	(54,059,000)	79,142,000	168,570,000
Cash flows from investing activities
Purchase of furniture, equipment and leasehold improvements	(4,528,000)	(10,024,000)	(8,672,000)
Payments to Acquire Intangible Assets	0	0	(400,000)
Proceeds from Life Insurance Policy	587,000	1,720,000	881,000
Cash used in investing activities	(3,941,000)	(8,304,000)	(8,191,000)
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	(18,578,000)	(5,944,000)	(5,833,000)
Payments of Ordinary Dividends, Noncontrolling Interest	0	0	(372,000)
Issuance of Class A non-voting common stock	56,000	83,000	70,000
Repurchase of Class A non-voting common stock for cancellation	(15,048,000)	(8,400,000)	(5,894,000)
Payment, Tax Withholding, Share-based Payment Arrangement	(5,839,000)	(1,014,000)	(2,529,000)
Proceeds from Issuance of Senior Long-term Debt	125,000,000	0	0
Redemption of senior secured notes	(148,574,000)	(50,000,000)	0
Repayments of Debt	1,426,000
Payments of Debt Issuance Costs	(1,210,000)	0	0
Payments For Debt Redemption Cost	(2,507,000)	(1,688,000)	0
Redemption of senior secured notes	82,000,000	(15,000,000)	(103,300,000)
Cash provided by (used in) financing activities	13,874,000	(81,963,000)	(117,858,000)
Net (decrease) increase in cash and cash equivalents	(44,126,000)	(11,125,000)	42,521,000
Cash and cash equivalents, beginning of year	79,550,000	90,675,000	48,154,000
Cash and cash equivalents, end of year	35,424,000	79,550,000	90,675,000
Schedule of non-cash financing activities
Employee share plan issuance	12,167,000	2,192,000	724,000
Supplemental disclosure of cash flow information
Cash paid during the year for interest	19,013,000	40,678,000	53,559,000
Cash paid during the year for income taxes, net	$ 11,191,000	$ 16,816,000	$ 11,520,000