Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net Revenues	$ 540,031	$ 406,601	$ 370,470	$ 387,247	$ 445,369	$ 386,314	$ 350,656	$ 257,713	$ 1,704,349	$ 1,440,052	$ 1,223,273
Operating Expenses									1,227,573	1,077,706	946,532
Other Expenses									47,965	101,172	148,071
Income Before Income from Equity Method Investments and Income Taxes	184,146	87,070	46,266	111,329	97,359	85,085	62,605	16,125	428,811	261,174	128,670
Income from Equity Method Investments	3,331	1,827	2,070	1,610	2,512	1,178	1,664	1,287	8,838	6,641	6,050
Pre-Tax Income (Loss)	187,477	$ 88,897	$ 48,336	$ 112,939	99,871	$ 86,263	$ 64,269	$ 17,412	437,649	267,815	134,720
Identifiable Segment Assets	1,584,886				1,662,346				1,584,886	1,662,346	1,479,171
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues									1,634,268	1,363,859	1,130,915
Operating Expenses									1,175,927	1,020,327	869,301
Other Expenses									35,810	92,172	108,739
Income Before Income from Equity Method Investments and Income Taxes									422,531	251,360	152,875
Income from Equity Method Investments									277	1,370	978
Pre-Tax Income (Loss)									422,808	252,730	153,853
Identifiable Segment Assets	1,294,103				1,302,351				1,294,103	1,302,351	1,097,373
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues									70,081	76,193	92,358
Operating Expenses									51,646	57,379	77,231
Other Expenses									12,155	9,000	39,332
Income Before Income from Equity Method Investments and Income Taxes									6,280	9,814	(24,205)
Income from Equity Method Investments									8,561	5,271	5,072
Pre-Tax Income (Loss)									14,841	15,085	(19,133)
Identifiable Segment Assets	$ 290,783				$ 359,995				$ 290,783	$ 359,995	$ 381,798