Acquisitions (Narrative) (Details) - USD ($) $ in Thousands							12 Months Ended
	Dec. 06, 2019	Nov. 01, 2019	Sep. 27, 2019	Sep. 03, 2019	Jul. 01, 2019	Jan. 02, 2019	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill							$ 1,181,998	$ 1,194,074
Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill							846,989	849,093
GMP Capital Inc. [Member]
Business Acquisition [Line Items]
Date of acquisition	Dec. 06, 2019
Payments to Acquire Intangible Assets							10,700
GMP Capital Inc. [Member] | Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill	$ 20,000
MainFirst Bank AG [Member]
Business Acquisition [Line Items]
Date of acquisition		Nov. 01, 2019
MainFirst Bank AG [Member] | Tradename, Non-Compete Agreements and Customer Relationships [Member]
Business Acquisition [Line Items]
Identifiable intangible assets		$ 800
MainFirst Bank AG [Member] | Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill		$ 15,700
George K. Baum & Company [Member]
Business Acquisition [Line Items]
Date of acquisition			Sep. 27, 2019
George K. Baum & Company [Member] | Customer Relationships and Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Identifiable intangible assets			$ 15,200
George K. Baum & Company [Member] | Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill			$ 33,300
B&F Capital Markets, Inc. [Member]
Business Acquisition [Line Items]
Date of acquisition				Sep. 03, 2019
Period of contingent consideration				5 years
Liability for earn-out payments							$ 16,000	20,100
B&F Capital Markets, Inc. [Member] | Tradename, Non-Compete Agreements and Customer Relationships [Member]
Business Acquisition [Line Items]
Identifiable intangible assets				$ 18,600
B&F Capital Markets, Inc. [Member] | Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill				$ 23,000
Mooreland Partners [Member]
Business Acquisition [Line Items]
Date of acquisition					Jul. 01, 2019		Jul. 01, 2019
Period of contingent consideration					3 years
Liability for earn-out payments							$ 13,000	$ 18,400
Mooreland Partners [Member] | Non-Compete Agreements and Backlog [Member]
Business Acquisition [Line Items]
Identifiable intangible assets					$ 5,100
Mooreland Partners [Member] | Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill					$ 45,800
First Empire Holding Corp. [Member]
Business Acquisition [Line Items]
Date of acquisition						Jan. 02, 2019
First Empire Holding Corp. [Member] | Tradename and Customer Relationships [Member]
Business Acquisition [Line Items]
Identifiable intangible assets						$ 7,800
First Empire Holding Corp. [Member] | Institutional Group [Member]
Business Acquisition [Line Items]
Goodwill						$ 14,900