Acquisition (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 01, 2017
Business Acquisition [Line Items]
Goodwill/(Bargain purchase gain)			$ 0	$ 7,946
Restructuring costs	$ 0	$ 222	$ 0	8,763
Convergex Group
Business Acquisition [Line Items]
Total estimated purchase price					$ 98,000
Business combination, recognized identifiable assets acquired and liabilities assumed, net						$ 109,600
Goodwill/(Bargain purchase gain)					11,600
Other tax expense (benefit)				$ 4,700
Restructuring costs					$ 8,800