Acquisition (Intangible Assets) (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Total intangible assets		$ 63,036	$ 55,187
Amortization of Intangible Assets		6,100	5,000	$ 2,700
Convergex Group [Member]
Business Acquisition [Line Items]
Trade name	$ 760
Intellectual property	1,790
Customer relationships	7,720
Total intangible assets	$ 10,270
Amortization of Intangible Assets		1,400
Intellectual Property [Member] | Convergex Group [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years
Trade Names
Business Acquisition [Line Items]
Total intangible assets		10,622	9,962
Customer Relationships [Member]
Business Acquisition [Line Items]
Total intangible assets		$ 41,134	$ 33,874
Customer Relationships [Member] | Convergex Group [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	9 years
Minimum | Convergex Group [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 months 6 days
Minimum | Trade Names
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 months 6 days
Minimum | Trade Names | Convergex Group [Member]
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 [Member]
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life	9 years
Maximum | Trade Names
Business Acquisition [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 years 6 months
Maximum | Trade Names | Convergex Group [Member]
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