Acquisition (Net assets acquired) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Jun. 01, 2017
Business Acquisition [Line Items]
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed	$ 1,100	$ 5,800
Convergex Group [Member]
Business Acquisition [Line Items]
Intangible assets, net			$ 10,300
Total estimated purchase price		96,200
Scenario, Plan [Member] | Convergex Group [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			31,780
Segregated cash			127,807
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, net			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)		13,343
Total estimated purchase price		$ 96,228