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
	Mar. 31, 2017	Mar. 31, 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	$ 3,627	$ 3,251
Income Taxes
Unrecognized tax benefits	0	0
Unrecognized tax benefits, interest and penalties	$ 0	$ 0
ASU 2016-09 | Adjusted | Deferred Tax Assets
Equity-based Compensation
Tax effect of adjustment			$ 700
ASU 2016-09 | Adjusted | Retained Earnings (Accumulated Deficit)
Equity-based Compensation
Cumulative effect on retained earnings and APIC			(4,900)
Tax effect of adjustment			700
ASU 2016-09 | Adjusted | Additional Paid-In Capital
Equity-based Compensation
Cumulative effect on retained earnings and APIC			$ 4,900
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