Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2018
Accounting Policies [Line Items]
Cash	$ 38,387			$ 52,373			$ 38,387	$ 52,373			$ 59,705
Cash equivalents	10,556			34,521			10,556	34,521			201,395
Account receivables	21,290						21,290				26,738
Interest Income	241			181			761	388
Retained earnings (accumulated deficit)	(316,122)						$ (316,122)				$ (237,782)
Accumulated other comprehensive income	(322)	$ (57)	$ 540	(707)	$ (1,843)	$ 1,679
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%
Income Taxes
Unrecognized tax benefits	0			$ 0			$ 0	$ 0
Assets and Liabilities, Lessee
Operating Lease, Liability	54,188						54,188
Operating lease, right-of-use asset	$ 47,910						$ 47,910
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
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
ASU 2016-02
Assets and Liabilities, Lessee
Operating Lease, Liability										$ 63,252
Operating lease, right-of-use asset										63,252
ASU 2016-02 | Adjustment For Prepaid Rent
Assets and Liabilities, Lessee
Operating lease, right-of-use asset										1,666
ASU 2016-02 | Adjustment For Accrued Rent
Assets and Liabilities, Lessee
Operating lease, right-of-use asset										$ 7,139
Minimum
Accounting Policies [Line Items]
Installment Period							3 years
Minimum | Office Equipment and Furniture and Fixtures
Accounting Policies [Line Items]
Useful lives							3 years
Maximum
Accounting Policies [Line Items]
Installment Period							4 years
Maximum | Office Equipment and Furniture and Fixtures
Accounting Policies [Line Items]
Useful lives							7 years