Acquisition (Net assets acquired) (Details) - Convergex Group [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Jun. 01, 2017
Business Acquisition [Line Items]
Intangible assets		$ 10,300
Total estimated purchase price	$ 98,000
Scenario, Plan [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		30,562
Segregated cash		129,025
Securities owned, at fair value		3,417
Securities borrowed		258,321
Deposits from clearing organizations		52,596
Receivable from brokers		62,983
Receivable from customers		43,469
Fees receivable		36,232
Fixed assets, net		1,325
Intangible assets		10,270
Other assets		7,528
Securities sold, not yet purchased, at fair value		(71)
Securities loaned		(286,359)
Payable to brokers		(74,284)
Payable to customers		(11,801)
Commission management payable		(82,718)
Compensation payable		(31,083)
Notes payable and other debt		(857)
Fees payable		(5,414)
Accounts payable, accrued expenses and other liabilities		(33,570)
Total identifiable net assets acquired and liabilities assumed		$ 109,571
Goodwill/(Bargain purchase gain)	11,609
Total estimated purchase price	$ 97,962