Consolidated Statements of Financial Condition - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
ASSETS
Cash and cash equivalents	$ 364,567	$ 506,735
Cash segregated under federal regulations	50,187	50,159
Investments, at fair value	35,875	36,078
Accounts receivable, net of allowance of $82 and $163 as of September 30, 2021 and December 31, 2020, respectively	75,520	63,881
Receivables from broker-dealers, clearing organizations and customers	721,127	408,346
Goodwill	154,789	154,789
Intangible assets, net of accumulated amortization	111,620	116,377
Furniture, equipment, leasehold improvements and capitalized software, net of accumulated depreciation and amortization	95,302	96,061
Operating lease right-of-use assets	69,189	70,960
Prepaid expenses and other assets	29,309	27,066
Total assets	1,707,485	1,530,452
Liabilities
Accrued employee compensation	26,166	59,719
Payables to broker-dealers, clearing organizations and customers	458,476	229,325
Income and other tax liabilities	42,614	40,456
Accounts payable, accrued expenses and other liabilities	70,297	71,218
Operating lease liabilities	86,391	88,425
Total liabilities	683,944	489,143
Commitments and Contingencies (Note 13)
Stockholders' equity
Preferred stock
Common stock	123	123
Additional paid-in capital	318,119	330,262
Treasury stock - Common stock voting, at cost, 2,880,856 and 2,845,770 shares as of September 30, 2021 and December 31, 2020, respectively	(271,512)	(232,712)
Retained earnings	995,192	956,966
Accumulated other comprehensive loss	(18,381)	(13,330)
Total stockholders' equity	1,023,541	1,041,309		$ 955,061
Total liabilities and stockholders' equity	1,707,485	1,530,452
Series A Preferred Stock [Member]
Stockholders' equity
Preferred stock