Segment Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net Revenues	$ 927,308	$ 402,515	$ 507,075	$ 427,007	$ 660,127	$ 402,198	$ 531,046	$ 415,327	$ 2,263,905	$ 2,008,698	$ 2,064,705
Operating Expenses									1,688,015	1,534,122	1,492,241
Other Expenses									49,457	36,865	30,387
Income Before Income from Equity Method Investments and Income Taxes	326,715	63,686	86,729	49,303	156,723	70,344	126,834	83,810	526,433	437,711	542,077
Income from Equity Method Investments	5,846	3,111	2,313	3,128	3,770	2,562	2,453	2,211	14,398	10,996	9,294
Pre-Tax Income	332,561	$ 66,797	$ 89,042	$ 52,431	160,493	$ 72,906	$ 129,287	$ 86,021	540,831	448,707	551,371
Identifiable Segment Assets	3,370,888				2,598,613				3,370,888	2,598,613	2,125,667
Investment Banking [Member]
Segment Reporting Information [Line Items]
Net Revenues									2,217,386	1,951,795	2,012,023
Operating Expenses									1,637,542	1,485,477	1,448,301
Other Expenses									49,112	33,618	30,366
Income Before Income from Equity Method Investments and Income Taxes									530,732	432,700	533,356
Income from Equity Method Investments									1,546	916	518
Pre-Tax Income									532,278	433,616	533,874
Identifiable Segment Assets	3,186,864				2,393,647				3,186,864	2,393,647	1,923,783
Investment Management [Member]
Segment Reporting Information [Line Items]
Net Revenues									46,519	56,903	52,682
Operating Expenses									50,473	48,645	43,940
Other Expenses									345	3,247	21
Income Before Income from Equity Method Investments and Income Taxes									(4,299)	5,011	8,721
Income from Equity Method Investments									12,852	10,080	8,776
Pre-Tax Income									8,553	15,091	17,497
Identifiable Segment Assets	$ 184,024				$ 204,966				$ 184,024	$ 204,966	$ 201,884