Debt and Convertible Preferred Stock Warrants - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 26, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013
Debt Disclosure [Line Items]
Debt, success fee			$ 300,000
Interest expense related to term loans			$ 400,000	$ 100,000	$ 500,000	$ 300,000
Letter of Credit
Debt Disclosure [Line Items]
Deb maturity date							Aug. 01, 2019
Line of credit							$ 1,500,000
Series G Convertible Preferred Stock
Debt Disclosure [Line Items]
Debt, success fee		$ 100,000
Series B Convertible Preferred Stock
Debt Disclosure [Line Items]
Outstanding preferred stock warrants exercised (in shares)			131,652		131,652
Exercise price of warrants (in dollars per share)			$ 1.03		$ 1.03
Series C Convertible Preferred Stock
Debt Disclosure [Line Items]
Outstanding preferred stock warrants exercised (in shares)			6,868		6,868
Exercise price of warrants (in dollars per share)			$ 2.18		$ 2.18
Series D Convertible Preferred Stock
Debt Disclosure [Line Items]
Outstanding preferred stock warrants exercised (in shares)			43,010		43,010
Exercise price of warrants (in dollars per share)			$ 2.79		$ 2.79
Series E Convertible Preferred Stock
Debt Disclosure [Line Items]
Outstanding preferred stock warrants exercised (in shares)			25,597		25,597
Exercise price of warrants (in dollars per share)			$ 2.93		$ 2.93
Convertible Preferred Stock
Debt Disclosure [Line Items]
Warrant term					10 years
Term Loan Facility
Debt Disclosure [Line Items]
Maximum borrowing capacity									$ 5,000,000
Increase in borrowing capacity					$ 11,600,000
Debt instrument required covenants percentage on forecasted gross revenue					80.00%
Debt, combined success fee							$ 300,000
Debt, additional success fee						100,000
Net cash proceeds from subordinated debt securities						$ 50,000,000
Term Loans
Debt Disclosure [Line Items]
Long-term debt, gross								$ 15,000,000
Term Loans | Subsequent Event
Debt Disclosure [Line Items]
Repayments on loans	$ 6,500,000
Term Loan I
Debt Disclosure [Line Items]
Long-term debt, gross								$ 7,500,000
Deb maturity date							Jan. 31, 2021	Jan. 31, 2019
Number of equal monthly installments					30 equal monthly installments
Variable annual interest rate							0.35%	0.10%
Term Loan II
Debt Disclosure [Line Items]
Long-term debt, gross								$ 7,500,000
Deb maturity date								Jan. 01, 2020