Acquisition (Narrative) (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Amortization of Intangible Assets		$ 5,000	$ 6,100	$ 5,000
Restructuring costs		0	8,763	0
Goodwill/(Bargain purchase gain)		0	6,914	$ 0
Finite-Lived Intangible Assets, Gross		63,036	63,036
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		4,162
Finite-Lived Intangible Assets, Amortization Expense, Year Two		3,950
Finite-Lived Intangible Assets, Amortization Expense, Year Three		3,890
Finite-Lived Intangible Assets, Amortization Expense, Year Four		3,108
Finite-Lived Intangible Assets, Amortization Expense, Year Five		2,768
Finite-Lived Intangible Assets, Amortization Expense, after Year Five		7,065
Finite-Lived Intangible Assets, Net		24,943
Convergex Group
Business Acquisition [Line Items]
Amortization of Intangible Assets		1,300	1,400
Other tax expense (benefit)			4,700
Restructuring costs			8,800
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed			6,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 10,300
Business combination, recognized identifiable assets acquired and liabilities assumed, Net	109,600
Goodwill/(Bargain purchase gain)			11,600
Total estimated purchase price			98,000
Finite-Lived Trade Names, Gross	760
Finite Lived Intellectual Property, Gross	1,790
Finite-Lived Customer Relationships, Gross	7,720
Finite-Lived Intangible Assets, Gross	10,270
Scenario, Plan [Member] | Convergex Group
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	30,562
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Segregated Cash	129,025
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities	3,417
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Assets	258,321
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	52,596
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Receivable from Broker	62,983
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Receivable from Customer	43,469
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Fees Receivable	36,232
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	1,325
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	10,270
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets	7,528
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Securities Sold Not Yet Purchased	71
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Financial Liabilities	286,359
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Payable to Broker	74,284
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Payable to Customers	11,801
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Commission Management Payable	82,718
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Compensation Payable	31,083
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	857
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Fees Payable	5,414
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities	33,570
Business combination, recognized identifiable assets acquired and liabilities assumed, Net	$ 109,571
Goodwill/(Bargain purchase gain)			11,609
Total estimated purchase price			97,962
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		1,239
Finite-Lived Intangible Assets, Amortization Expense, Year Two		1,216
Finite-Lived Intangible Assets, Amortization Expense, Year Three		1,216
Finite-Lived Intangible Assets, Amortization Expense, Year Four		1,007
Finite-Lived Intangible Assets, Amortization Expense, Year Five		858
Finite-Lived Intangible Assets, Amortization Expense, after Year Five		2,073
Finite-Lived Intangible Assets, Net		7,609
Intellectual Property [Member] | Convergex Group
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Trade Names [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross		10,622	10,622
Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Gross		$ 41,134	$ 41,134
Customer Relationships [Member] | Convergex Group
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	9 years
Minimum | Convergex Group
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 months 6 days
Minimum | Trade Names [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 months 6 days
Minimum | Trade Names [Member] | Convergex Group
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 months 6 days
Minimum | Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life		3 years
Maximum | Convergex Group
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	9 years
Maximum | Trade Names [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 years 6 months
Maximum | Trade Names [Member] | Convergex Group
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	2 years 7 months 6 days
Maximum | Customer Relationships [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life		14 years