Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net Revenues	$ 771,406	$ 381,259	$ 448,477	$ 463,563	$ 540,031	$ 406,601	$ 370,470	$ 387,247	$ 2,064,705	$ 1,704,349	$ 1,440,052
Operating Expenses									1,492,241	1,227,573	1,077,706
Other Expenses									30,387	47,965	101,172
Income Before Income from Equity Method Investments and Income Taxes	250,206	74,540	104,782	112,549	184,146	87,070	46,266	111,329	542,077	428,811	261,174
Income from Equity Method Investments	2,452	2,298	2,419	2,125	3,331	1,827	2,070	1,610	9,294	8,838	6,641
Pre-Tax Income	252,658	$ 76,838	$ 107,201	$ 114,674	187,477	$ 88,897	$ 48,336	$ 112,939	551,371	437,649	267,815
Identifiable Segment Assets	2,125,667				1,584,886				2,125,667	1,584,886	1,662,346
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues									2,012,023	1,634,268	1,363,859
Operating Expenses									1,448,301	1,175,927	1,020,327
Other Expenses									30,366	35,810	92,172
Income Before Income from Equity Method Investments and Income Taxes									533,356	422,531	251,360
Income from Equity Method Investments									518	277	1,370
Pre-Tax Income									533,874	422,808	252,730
Identifiable Segment Assets	1,923,783				1,294,103				1,923,783	1,294,103	1,302,351
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues									52,682	70,081	76,193
Operating Expenses									43,940	51,646	57,379
Other Expenses									21	12,155	9,000
Income Before Income from Equity Method Investments and Income Taxes									8,721	6,280	9,814
Income from Equity Method Investments									8,776	8,561	5,271
Pre-Tax Income									17,497	14,841	15,085
Identifiable Segment Assets	$ 201,884				$ 290,783				$ 201,884	$ 290,783	$ 359,995