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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Equipment, Deferred Tax Asset, Revenue and Expense Recognition, Equity-Based Compensation and Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jan. 01, 2017
Deferred Tax Asset and Amount Due Pursuant to Tax Receivable Agreement
Percentage of tax benefits payable to partners under tax receivable agreement			85.00%
Remaining percentage of cash savings realized by the Company (as a percent)			15.00%
Revenue and Expense Recognition
Reimbursable expenses billed to clients	$ 4,248	$ 4,051	$ 10,583	$ 11,445
Income Taxes
Unrecognized tax benefits	$ 0	$ 0	$ 0	$ 0
ASU 2016-09 \| Adjusted \| Deferred Tax Assets
Equity-based Compensation
Tax effect of adjustment					$ 658
ASU 2016-09 \| Adjusted \| Retained Earnings (Accumulated Deficit)
Equity-based Compensation
Cumulative effect on retained earnings and APIC					(4,855)
Tax effect of adjustment					658
ASU 2016-09 \| Adjusted \| Additional Paid-In Capital
Equity-based Compensation
Cumulative effect on retained earnings and APIC					$ 4,855
Issued Prior to December 1, 2016
Equity-based Compensation
Minimum age of retiring employees required so that certain qualifying awards granted during employment will not be forfeited			54 years
Consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited			8 years
Issued On or after December 1, 2016
Equity-based Compensation
Minimum age of retiring employees required so that certain qualifying awards granted during employment will not be forfeited			56 years
Consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited			5 years
Minimum total age and consecutive years of service of retiring employees required so that certain qualifying awards granted during employment will not be forfeited			65 years
Office equipment and furniture and fixtures \| Minimum
Equipment and leasehold improvements, net
Useful lives			3 years
Office equipment and furniture and fixtures \| Maximum
Equipment and leasehold improvements, net
Useful lives			7 years