Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net Revenues	$ 660,127	$ 402,198	$ 531,046	$ 415,327	$ 771,406	$ 381,259	$ 448,477	$ 463,563	$ 2,008,698	$ 2,064,705	$ 1,704,349
Operating Expenses									1,534,122	1,492,241	1,227,573
Other Expenses									36,865	30,387	47,965
Income Before Income from Equity Method Investments and Income Taxes	156,723	70,344	126,834	83,810	250,206	74,540	104,782	112,549	437,711	542,077	428,811
Income from Equity Method Investments	3,770	2,562	2,453	2,211	2,452	2,298	2,419	2,125	10,996	9,294	8,838
Pre-Tax Income	160,493	$ 72,906	$ 129,287	$ 86,021	252,658	$ 76,838	$ 107,201	$ 114,674	448,707	551,371	437,649
Identifiable Segment Assets	2,598,613				2,125,667				2,598,613	2,125,667	1,584,886
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues									1,951,795	2,012,023	1,634,268
Operating Expenses									1,485,477	1,448,301	1,175,927
Other Expenses									33,618	30,366	35,810
Income Before Income from Equity Method Investments and Income Taxes									432,700	533,356	422,531
Income from Equity Method Investments									916	518	277
Pre-Tax Income									433,616	533,874	422,808
Identifiable Segment Assets	2,393,647				1,923,783				2,393,647	1,923,783	1,294,103
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues									56,903	52,682	70,081
Operating Expenses									48,645	43,940	51,646
Other Expenses									3,247	21	12,155
Income Before Income from Equity Method Investments and Income Taxes									5,011	8,721	6,280
Income from Equity Method Investments									10,080	8,776	8,561
Pre-Tax Income									15,091	17,497	14,841
Identifiable Segment Assets	$ 204,966				$ 201,884				$ 204,966	$ 201,884	$ 290,783