Segment Operating Results - (Footnotes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Other Revenue, net	$ 67,496	$ 147	$ (5,716)
Adjustments to Tax Receivable Agreements	77,535	0	0
Gain (Loss) on Disposition of Business	7,808	0	0
Special Charges	25,437	8,100	41,144
Acquisition and Transition Costs	1,673	99	4,890
Total Other Expenses	47,965	101,172	148,071
Investment Banking [Member]
Segment Reporting Information [Line Items]
Other Revenue, net	59,100	(239)	(2,945)
Interest expense on Notes Payable, Subordinated Borrowing and Line of Credit	9,960	9,578	6,041
Adjustments to Tax Receivable Agreements	77,535
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	16,266
Amortization of LP Units / Interests and Certain Other Awards	11,444	80,846	83,673
Acquisition Related Compensation Charges	0	0	1,537
Special Charges	14,400	0	2,151
Acquisition and Transition Costs	555	(692)	4,879
Fair Value of Contingent Consideration	0	1,107	2,704
Intangible Asset and Other Amortization	9,411	10,911	13,795
Total Other Expenses	35,810	92,172	108,739
Investment Management [Member]
Segment Reporting Information [Line Items]
Other Revenue, net	8,396	386	(2,771)
Interest expense on Notes Payable, Subordinated Borrowing and Line of Credit		670	3,576
Gain (Loss) on Disposition of Business	7,808
Special Charges	11,037	8,100	38,993
Acquisition and Transition Costs	1,118	791	11
Intangible Asset and Other Amortization	0	109	328
Total Other Expenses	$ 12,155	$ 9,000	$ 39,332