Acquisition (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 01, 2017	Sep. 23, 2016	May 06, 2016
Business Acquisition [Line Items]
Amortization of Intangible Assets	$ 6,100,000	$ 5,000,000	$ 2,700,000
Restructuring Costs	8,763,000	0	0
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low	100,000	100,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	15,000,000	15,100,000
Proceeds from divested business, net of cash divested	0	17,303,000	$ 0
Convergex Group [Member]
Business Acquisition [Line Items]
Total estimated purchase price	98,000,000
Payments to Acquire Businesses, Gross	48,600,000
Business Acquisition, Share Price				$ 15.05
Intangible assets, net				$ 10,300,000
Amortization of Intangible Assets	1,400,000
Other Tax Expense (Benefit)	4,700,000
Restructuring Costs	8,800,000
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed	6,000,000
CRT business
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		6,300,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low						$ 0
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Cost Expensed		400,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High						8,000,000
Contractual Obligation						$ 6,000,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	33,200,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	$ 9,300,000
Common Stock Class A | Convergex Group [Member]
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	3,162,278
Scenario, Plan [Member] | Convergex Group [Member]
Business Acquisition [Line Items]
Total estimated purchase price	$ 97,962,000
Intangible assets, net				10,270,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net				$ 109,571,000
Goodwill/(Bargain purchase gain)	$ 11,609,000
Alternative Solutions [Member]
Business Acquisition [Line Items]
Assets under Management, Carrying Amount					$ 2,500,000,000
Proceeds from divested business, net of cash divested		$ 17,300,000