Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net Revenues	$ 531,046	$ 448,477	$ 946,373	$ 912,040
Operating Expenses	397,303	337,815	721,454	680,771
Other Expenses	6,909	5,880	14,275	13,938
Income Before Income from Equity Method Investments and Income Taxes	126,834	104,782	210,644	217,331
Income from Equity Method Investments	2,453	2,419	4,664	4,544
Pre-Tax Income	129,287	107,201	215,308	221,875
Identifiable Segment Assets	1,939,749	1,612,524	1,939,749	1,612,524	$ 2,125,667
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues	516,386	435,675	917,574	885,875
Operating Expenses	385,378	327,137	697,288	658,820
Other Expenses	6,909	5,880	14,167	13,917
Income Before Income from Equity Method Investments and Income Taxes	124,099	102,658	206,119	213,138
Income from Equity Method Investments	219	297	474	297
Pre-Tax Income	124,318	102,955	206,593	213,435
Identifiable Segment Assets	1,749,339	1,354,810	1,749,339	1,354,810
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues	14,660	12,802	28,799	26,165
Operating Expenses	11,925	10,678	24,166	21,951
Other Expenses	0	0	108	21
Income Before Income from Equity Method Investments and Income Taxes	2,735	2,124	4,525	4,193
Income from Equity Method Investments	2,234	2,122	4,190	4,247
Pre-Tax Income	4,969	4,246	8,715	8,440
Identifiable Segment Assets	$ 190,410	$ 257,714	$ 190,410	$ 257,714