Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash and cash equivalents	$ 79,550	$ 90,675	$ 48,154	$ 64,913
Deposits with clearing organizations	48,415	67,678
Receivable from brokers, dealers and clearing organizations	163,293	166,493
Receivable from customers, net of allowance for credit losses of $451 ($886 in 2018)	796,934	720,777
Income tax receivable	5,170	1,014
Securities purchased under agreements to resell	0	290
Securities owned, at fair value	799,719	837,584
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $38,355 and $3,673, respectively ($25,109 and $6,800, respectively, in 2018)	43,670	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	31,377	28,988
Subordinated loan receivable	0	0
Right-of-use lease assets, net of accumulated amortization of $25,186	160,297	0
Intangible assets	32,100	32,100
Goodwill	137,889	137,889
Other assets	166,341	112,768
Deferred tax assets	0	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,464,755	2,240,314
Liabilities
Drafts payable	0	16,348
Bank call loans	0	15,000
Payable to brokers, dealers and clearing organizations	520,975	289,207
Payable to customers	334,735	336,616
Securities sold under agreements to repurchase	287,265	484,218
Securities sold but not yet purchased, at fair value	100,571	85,446
Accrued compensation	207,358	167,348
Accounts payable and other liabilities	44,725	87,630
Lease liabilities	203,140	0
Income tax payable	0	0
Senior secured notes, net of debt issuance costs of $485 ($904 in 2018)	149,515	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	23,749	14,083
Intercompany payables	0	0
Total liabilities	1,872,033	1,694,992
Stockholders' equity
Stockholders' equity	592,722	545,322
Total stockholders' equity			523,911
Total liabilities and stockholders' equity	2,464,755	2,240,314
Allowance for credit losses	451	886
Amounts pledged	357,100	518,000
Notes Receivable, Net Accumulated Amortization	38,355	25,109
Notes Receivable, Net Allowance for Uncollectibles	3,673	6,800
Net accumulated depreciation	94,773	89,182
Unamortized debt issuance expense	485	904
Eliminations
ASSETS
Cash and cash equivalents	0	0	0	0
Deposits with clearing organizations	0	0
Receivable from brokers, dealers and clearing organizations	5,062	0
Receivable from customers, net of allowance for credit losses of $451 ($886 in 2018)	0	0
Income tax receivable	(53,855)	(67,508)
Securities purchased under agreements to resell		0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $38,355 and $3,673, respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0	0
Subordinated loan receivable	(112,767)	(112,558)
Right-of-use lease assets, net of accumulated amortization of $25,186	0
Intangible assets	0	0
Goodwill	0	0
Other assets	(10,432)	0
Deferred tax assets	(9,501)	(18,950)
Investment in subsidiaries	(1,485,739)	(1,208,541)
Intercompany receivables	(70,798)	(26,330)
Total assets	(1,738,030)	(1,433,887)
Liabilities
Drafts payable		0
Bank call loans		0
Payable to brokers, dealers and clearing organizations	0	0
Payable to customers	0	0
Securities sold under agreements to repurchase	0	0
Securities sold but not yet purchased, at fair value	0	0
Accrued compensation	0	0
Accounts payable and other liabilities	(10,565)	(9)
Lease liabilities	0
Income tax payable	(743)	(67,507)
Senior secured notes, net of debt issuance costs of $485 ($904 in 2018)	0	0
Subordinated indebtedness	(112,558)	(112,558)
Deferred tax liabilities, net	(9,797)	(18,946)
Intercompany payables	(299)	(26,334)
Total liabilities	(133,962)	(225,354)
Stockholders' equity
Stockholders' equity	(1,604,068)	(1,208,533)
Total liabilities and stockholders' equity	(1,738,030)	(1,433,887)
Parent
ASSETS
Cash and cash equivalents	4,811	53,526	7,442	229
Deposits with clearing organizations	0	0
Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers, net of allowance for credit losses of $451 ($886 in 2018)	0	0
Income tax receivable	42,556	45,733
Securities purchased under agreements to resell		0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $38,355 and $3,673, respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0	0
Subordinated loan receivable	209	0
Right-of-use lease assets, net of accumulated amortization of $25,186	0
Intangible assets	0	0
Goodwill	0	0
Other assets	154	135
Deferred tax assets	4	1
Investment in subsidiaries	697,093	661,837
Intercompany receivables	2,875	(14,211)
Total assets	747,702	747,021
Liabilities
Drafts payable		0
Bank call loans		0
Payable to brokers, dealers and clearing organizations	0	0
Payable to customers	0	0
Securities sold under agreements to repurchase	0	0
Securities sold but not yet purchased, at fair value	0	0
Accrued compensation	0	0
Accounts payable and other liabilities	5,166	163
Lease liabilities	0
Income tax payable	0	2,440
Senior secured notes, net of debt issuance costs of $485 ($904 in 2018)	149,515	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	299	0
Total liabilities	154,980	201,699
Stockholders' equity
Stockholders' equity	592,722	545,322
Total liabilities and stockholders' equity	747,702	747,021
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	2,179	3,826	3,716	10,284
Deposits with clearing organizations	0	0
Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers, net of allowance for credit losses of $451 ($886 in 2018)	0	0
Income tax receivable	16,469	23,491
Securities purchased under agreements to resell		0
Securities owned, at fair value	1,352	1,358
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $38,355 and $3,673, respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	22,537	20,722
Subordinated loan receivable	112,558	112,558
Right-of-use lease assets, net of accumulated amortization of $25,186	153,780
Intangible assets	400	400
Goodwill	0	0
Other assets	11,798	2,581
Deferred tax assets	7,048	455
Investment in subsidiaries	763,990	546,704
Intercompany receivables	67,923	46,840
Total assets	1,160,034	758,935
Liabilities
Drafts payable		0
Bank call loans		0
Payable to brokers, dealers and clearing organizations	0	0
Payable to customers	0	0
Securities sold under agreements to repurchase	0	0
Securities sold but not yet purchased, at fair value	0	0
Accrued compensation	0	0
Accounts payable and other liabilities	75	31,653
Lease liabilities	196,234
Income tax payable	40	22,189
Senior secured notes, net of debt issuance costs of $485 ($904 in 2018)	0	0
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	0	26,334
Total liabilities	196,349	80,176
Stockholders' equity
Stockholders' equity	963,685	678,759
Total liabilities and stockholders' equity	1,160,034	758,935
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	72,560	33,323	$ 36,996	$ 54,400
Deposits with clearing organizations	48,415	67,678
Receivable from brokers, dealers and clearing organizations	158,231	166,493
Receivable from customers, net of allowance for credit losses of $451 ($886 in 2018)	796,934	720,777
Income tax receivable	0	(702)
Securities purchased under agreements to resell		290
Securities owned, at fair value	798,367	836,226
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $38,355 and $3,673, respectively ($25,109 and $6,800, respectively, in 2018)	43,670	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	8,840	8,266
Subordinated loan receivable	0	0
Right-of-use lease assets, net of accumulated amortization of $25,186	6,517
Intangible assets	31,700	31,700
Goodwill	137,889	137,889
Other assets	164,821	110,052
Deferred tax assets	2,449	18,494
Investment in subsidiaries	24,656	0
Intercompany receivables	0	(6,299)
Total assets	2,295,049	2,168,245
Liabilities
Drafts payable		16,348
Bank call loans		15,000
Payable to brokers, dealers and clearing organizations	520,975	289,207
Payable to customers	334,735	336,616
Securities sold under agreements to repurchase	287,265	484,218
Securities sold but not yet purchased, at fair value	100,571	85,446
Accrued compensation	207,358	167,348
Accounts payable and other liabilities	50,049	55,823
Lease liabilities	6,906
Income tax payable	703	42,878
Senior secured notes, net of debt issuance costs of $485 ($904 in 2018)	0	0
Subordinated indebtedness	112,558	112,558
Deferred tax liabilities, net	33,546	33,029
Intercompany payables	0	0
Total liabilities	1,654,666	1,638,471
Stockholders' equity
Stockholders' equity	640,383	529,774
Total liabilities and stockholders' equity	$ 2,295,049	$ 2,168,245