Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net Revenues	$ 463,563	$ 387,247
Operating Expenses	342,956	294,897
Other Expenses	8,058	(18,979)
Income Before Income from Equity Method Investments and Income Taxes	112,549	111,329
Income (Loss) from Equity Method Investments	2,125	1,610
Pre-Tax Income	114,674	112,939
Identifiable Segment Assets	1,417,653	1,432,762	$ 1,584,886
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues	450,200	370,770
Operating Expenses	331,683	281,592
Other Expenses	8,037	(18,979)
Income Before Income from Equity Method Investments and Income Taxes	110,480	108,157
Income (Loss) from Equity Method Investments	0	(149)
Pre-Tax Income	110,480	108,008
Identifiable Segment Assets	1,148,662	1,077,509
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues	13,363	16,477
Operating Expenses	11,273	13,305
Other Expenses	21	0
Income Before Income from Equity Method Investments and Income Taxes	2,069	3,172
Income (Loss) from Equity Method Investments	2,125	1,759
Pre-Tax Income	4,194	4,931
Identifiable Segment Assets	$ 268,991	$ 355,253