Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net Revenues	$ 448,477	$ 370,470	$ 912,040	$ 757,717
Operating Expenses	337,815	282,826	680,771	577,723
Other Expenses	5,880	41,378	13,938	22,399
Income Before Income from Equity Method Investments and Income Taxes	104,782	46,266	217,331	157,595
Income (Loss) from Equity Method Investments	2,419	2,070	4,544	3,680
Pre-Tax Income (Loss)	107,201	48,336	221,875	161,275
Identifiable Segment Assets	1,612,524	1,451,286	1,612,524	1,451,286	$ 1,584,886
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues	435,675	356,409	885,875	727,179
Operating Expenses	327,137	269,717	658,820	551,309
Other Expenses	5,880	33,894	13,917	14,915
Income Before Income from Equity Method Investments and Income Taxes	102,658	52,798	213,138	160,955
Income (Loss) from Equity Method Investments	297	113	297	(36)
Pre-Tax Income (Loss)	102,955	52,911	213,435	160,919
Identifiable Segment Assets	1,354,810	1,135,094	1,354,810	1,135,094
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues	12,802	14,061	26,165	30,538
Operating Expenses	10,678	13,109	21,951	26,414
Other Expenses	0	7,484	21	7,484
Income Before Income from Equity Method Investments and Income Taxes	2,124	(6,532)	4,193	(3,360)
Income (Loss) from Equity Method Investments	2,122	1,957	4,247	3,716
Pre-Tax Income (Loss)	4,246	(4,575)	8,440	356
Identifiable Segment Assets	$ 257,714	$ 316,192	$ 257,714	$ 316,192