Consolidated Statements of Cash Flows - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Cash Flows [Abstract]
Repurchase of Secured Debt	$ 1,426,000	$ 0
Net Income (Loss) Attributable to Parent	7,818,000	11,194,000
Non-cash items included in net income:
Depreciation and amortization of furniture, equipment and leasehold improvements	2,057,000	1,708,000
Deferred income taxes	3,019,000	1,894,000
Amortization of notes receivable	3,059,000	3,388,000
Amortization of debt issuance costs	49,000	64,000
Write-off of debt issuance costs	4,000	0
Provision for credit losses	358,000	12,000
Share-based compensation	(1,376,000)	3,042,000
Amortization of right-of-use lease assets	6,213,000	0
Gain on repurchase of senior secured notes	(85,560)	0
Decrease (increase) in operating assets:
Deposits with clearing organizations	71,165,000	(12,609,000)
Receivable from brokers, dealers and clearing organizations	(35,124,000)	(13,728,000)
Receivable from customers	(168,196,000)	(32,860,000)
Income tax receivable	(1,113,000)	1,014,000
Securities purchased under agreements to resell	0	(299,000)
Securities owned	490,373,000	(73,884,000)
Notes receivable	(4,349,000)	(3,251,000)
Other assets	49,744,000	(14,326,000)
Increase (decrease) in operating liabilities:
Drafts payable	19,066,000	(2,815,000)
Payable to brokers, dealers and clearing organizations	(281,392,000)	345,126,000
Payable to customers	78,911,000	(2,240,000)
Income taxes payable	0	1,162,000
Securities sold under agreements to repurchase	(163,162,000)	(215,597,000)
Securities sold but not yet purchased	(65,932,000)	51,840,000
Accrued compensation	(101,513,000)	(61,633,000)
Accounts payable and other liabilities	(8,286,000)	2,907,000
Cash (used in) provided by operating activities	(241,023,000)	15,327,000
Cash flows from investing activities
Purchase of furniture, equipment and leasehold improvements	(1,326,000)	(3,448,000)
Cash used in investing activities	(1,326,000)	(3,448,000)
Cash flows from financing activities
Cash dividends paid on Class A non-voting and Class B voting common stock	(1,561,000)	(1,432,000)
Proceeds from Issuance of Common Stock		(2,035,000)
Repurchase of Class A non-voting common stock for cancellation	(8,434,000)	(2,035,000)
Payments for employee taxes withheld related to vested share-based awards	(5,340,000)	(1,014,000)
Increase (decrease) in bank call loans, net	203,100,000	(15,000,000)
Cash provided by (used in) financing activities	186,339,000	(19,481,000)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	(56,010,000)	(7,602,000)
Schedule of non-cash financing activities
Employee share plan issuance	10,032,000	1,706,000
Supplemental disclosure of cash flow information
Interest Paid, Excluding Capitalized Interest, Operating Activities	9,095,000	9,385,000
Cash paid during the period for income taxes, net	516,000	792,000
Cash and cash equivalents	$ 23,540,000	$ 83,073,000