Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2018
Accounting Policies [Line Items]
Cash	$ 32,703	$ 76,791			$ 59,705
Cash equivalents	36,429	54,199			201,395
Account receivables	26,359				26,738
Interest Income	275	101
Retained earnings (accumulated deficit)	(327,648)				$ (237,782)
Accumulated other comprehensive income	$ 540	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%
Revenue and Expense Recognition and Equity-Based Compensation
Operating Lease, Liability	$ 59,333
Operating lease, right-of-use asset	54,385
Income Taxes
Unrecognized tax benefits	$ 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
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
Revenue and Expense Recognition and Equity-Based Compensation
Operating Lease, Liability				$ 63,252
Operating lease, right-of-use asset				63,252
ASU 2016-02 | Adjustment For Prepaid Rent
Revenue and Expense Recognition and Equity-Based Compensation
Operating lease, right-of-use asset				1,666
ASU 2016-02 | Adjustment For Accrued Rent
Revenue and Expense Recognition and Equity-Based Compensation
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