Acquisition (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 01, 2017
Business Acquisition [Line Items]
Other Tax Expense (Benefit)			$ 5,400
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed	$ 1,100		5,800
Restructuring Costs	222	$ 0	8,763	$ 0
Convergex Group [Member]
Business Acquisition [Line Items]
Total estimated purchase price			96,200
Payments to Acquire Businesses, Gross			48,600
Business Acquisition, Share Price					$ 15.05
Intangible assets, net					$ 10,300
Amortization of Intangible Assets	$ 600		$ 800
Common Stock Class A | Convergex Group [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares			3,162,278
Scenario, Plan [Member] | Convergex Group [Member]
Business Acquisition [Line Items]
Total estimated purchase price			$ 96,228
Intangible assets, net					10,270
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net					$ 109,571
Goodwill/(Bargain purchase gain)			$ 13,343