Convertible Preferred Stock and Stockholders' Deficit - Additional Information (Details) - USD ($)		12 Months Ended
	Jul. 02, 2018	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Stockholders Deficit [Line Items]
IPO closing date	Jul. 02, 2018
Net proceeds from initial public offering		$ 99,998,000
Common stock, par value per share		$ 0.01		$ 0.01
Common stock, shares authorized for issuance		200,000,000		200,000,000
Preferred stock, shares authorized for issuance		10,000,000		10,000,000
Preferred stock, par value per share		$ 0.01		$ 0.01
Common stock, shares issued		17,744,000		231,000
Common stock, shares outstanding		17,744,000		231,000
Convertible preferred stock, shares authorized		308,593,000		308,593,000
Convertible preferred stock, par value		$ 0.01		$ 0.01
Liquidation preference		$ 0		$ 108,324,000
Convertible preferred stock non-cumulative dividends rate		9.00%
Common stock voting rights		one
Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock converted into common stock		(304,958,000)
Convertible preferred stock, shares authorized			308,600,000
Convertible preferred stock, par value			$ 0.01
Oxford Finance LLC
Stockholders Deficit [Line Items]
Convertible preferred stock converted to common stock, ratio	2898.55%
Series D Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				49,426,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 14,205,000
Series E Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				44,873,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 22,876,000
Series F Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				105,567,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 34,343,000
Additional liquidation preference				$ 0.1678
Additional liquidation preference per share due				$ 1.0068
Series G Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				40,584,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 15,000,000
Series A-2 Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				25,385,000
Convertible preferred stock converted to common stock, ratio	2825.65%
Liquidation preference				$ 5,209,000
Series B Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				17,000,000
Convertible preferred stock converted to common stock, ratio	2437.24%
Liquidation preference				$ 5,386,000
Series C Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				20,958,000
Convertible preferred stock converted to common stock, ratio	1971.99%
Liquidation preference				$ 11,009,000
Junior Securities
Stockholders Deficit [Line Items]
Liquidation preference				$ 59,200,000
Common Stock
Stockholders Deficit [Line Items]
Common stock shares issued and sold	6,325,000	6,325,000
Convertible preferred stock converted into common stock	11,000,000	10,994,000
Initial Public Offering
Stockholders Deficit [Line Items]
IPO closing date	Jul. 02, 2018
Net proceeds from initial public offering	$ 96,500,000
Common stock, par value per share	$ 0.01
Common stock, shares authorized for issuance	200,000,000
Preferred stock, shares authorized for issuance	10,000,000
Preferred stock, par value per share	$ 0.01
Initial Public Offering | Common Stock
Stockholders Deficit [Line Items]
Common stock shares issued and sold	6,325,000
Issuance price per share	$ 17.00
Convertible preferred stock converted into common stock	11,000,000