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Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jan. 01, 2017
Accounting Policies [Line Items]
Cash	$ 52,373		$ 52,373	$ 59,271	$ 86,032
Cash equivalents	34,521		34,521	68,329	127,159
Retained earnings (accumulated deficit)	(250,594)		(250,594)		(139,918)
Accumulated other comprehensive income			$ (871)	2,234
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 and Equity-Based Compensation
Revenue recognized			$ 4,817
Income Taxes
Unrecognized tax benefits	0		$ 0	$ 0
Issued Prior to December 1, 2016
Revenue and Expense Recognition and 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
Revenue and Expense Recognition and 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
Accounting Policies [Line Items]
Useful lives			3 years
Office Equipment and Furniture and Fixtures \| Maximum
Accounting Policies [Line Items]
Useful lives			7 years
ASU 2016-01
Accounting Policies [Line Items]
Retained earnings (accumulated deficit)					(317)
Accumulated other comprehensive income					317
ASU 2014-09
Revenue and Expense Recognition and Equity-Based Compensation
Cumulative effect on retained earnings and APIC					3,722
Tax effect of adjustment					567
Net change resulting from cumulative effect on retained earnings and APIC					$ 3,155
Revenue recognized	16,244	$ 36,867	$ 16,244
Reimbursable expenses recognized	$ 4,356		$ 11,229
ASU 2016-09 \| Adjusted \| Deferred Tax Assets
Revenue and Expense Recognition and Equity-Based Compensation
Tax effect of adjustment						$ 658
ASU 2016-09 \| Adjusted \| Retained Earnings (Accumulated Deficit)
Revenue and Expense Recognition and 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
Revenue and Expense Recognition and Equity-Based Compensation
Cumulative effect on retained earnings and APIC						$ 4,855