Income Taxes - Details of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets:
Depreciation and Amortization	$ 33,738	$ 26,319
Compensation and Benefits	61,541	46,697
Other	24,720	20,282
Total Deferred Tax Assets	268,186	225,541
Deferred Tax Liabilities:
Goodwill, Intangible Assets and Other	18,873	20,241
Total Deferred Tax Liabilities	18,873	20,241
Net Deferred Tax Assets Before Valuation Allowance	249,313	205,300
Valuation Allowance	(8,221)	(6,406)
Net Deferred Tax Assets	241,092	198,894
Tax Receivable Agreement [Member]
Deferred Tax Assets:
Step up in tax basis due to the exchange of LP Units for Class A Shares	111,108	106,360
Without Tax Receivable Agreement [Member]
Deferred Tax Assets:
Step up in tax basis due to the exchange of LP Units for Class A Shares	$ 37,079	$ 25,883