Acquisition (Narrative) (Details) - Convergex Group [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Jun. 01, 2017
Business Acquisition [Line Items]
Other Tax Expense (Benefit)	$ 4.7
Restructuring Costs	8.8
Scenario, Plan [Member]
Business Acquisition [Line Items]
Total estimated purchase price	98.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net		$ 109.6
Goodwill/(Bargain purchase gain)	$ 11.6