Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net Revenues	$ 406,601	$ 386,314	$ 1,164,318	$ 994,683
Operating Expenses	307,291	283,509	885,014	754,203
Other Expenses	12,240	17,720	34,639	76,665
Income Before Income from Equity Method Investments and Income Taxes	87,070	85,085	244,665	163,815
Income (Loss) from Equity Method Investments	1,827	1,178	5,507	4,129
Pre-Tax Income	88,897	86,263	250,172	167,944
Identifiable Segment Assets	1,561,035	1,461,630	1,561,035	1,461,630	$ 1,662,346
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues	388,299	368,634	1,115,478	936,504
Operating Expenses	293,264	268,666	844,573	707,846
Other Expenses	11,748	17,422	26,663	75,854
Income Before Income from Equity Method Investments and Income Taxes	83,287	82,546	244,242	152,804
Income (Loss) from Equity Method Investments	(75)	(112)	(111)	(94)
Pre-Tax Income	83,212	82,434	244,131	152,710
Identifiable Segment Assets	1,239,812	1,114,565	1,239,812	1,114,565
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues	18,302	17,680	48,840	58,179
Operating Expenses	14,027	14,843	40,441	46,357
Other Expenses	492	298	7,976	811
Income Before Income from Equity Method Investments and Income Taxes	3,783	2,539	423	11,011
Income (Loss) from Equity Method Investments	1,902	1,290	5,618	4,223
Pre-Tax Income	5,685	3,829	6,041	15,234
Identifiable Segment Assets	$ 321,223	$ 347,065	$ 321,223	$ 347,065