Stockholders’ Deficit and Convertible Preferred Stock (Details) - $ / shares	May 06, 2019	Mar. 30, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, authorized (in shares)		60,000,000	58,669,600
Common stock, par value (in dollars per share)		$ 0.0001	$ 0.0001
Common stock, issued (in shares)		7,120,933	6,951,350
Common stock, outstanding (in shares)		7,120,933	6,951,350
Convertible preferred stock, authorized (in shares)		43,882,867	43,882,867
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, outstanding (in shares)		41,562,111	41,562,111	39,474,156	39,361,211
Series A
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		3,333,500	3,333,500
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		3,333,500	3,333,500
Convertible preferred stock, outstanding (in shares)		3,333,500	3,333,500
Series B
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		4,802,260	4,802,260
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		4,680,565	4,680,565
Convertible preferred stock, outstanding (in shares)		4,680,565	4,680,565
Series C
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		8,076,643	8,076,643
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		8,076,636	8,076,636
Convertible preferred stock, outstanding (in shares)		8,076,636	8,076,636
Series D
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		8,713,207	8,713,207
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		8,713,201	8,713,201
Convertible preferred stock, outstanding (in shares)		8,713,201	8,713,201
Series E
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		4,740,531	4,740,531
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		4,701,449	4,701,449
Convertible preferred stock, outstanding (in shares)		4,701,449	4,701,449
Series F
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		4,866,776	4,866,776
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		4,866,758	4,866,758
Convertible preferred stock, outstanding (in shares)		4,866,758	4,866,758
Series G
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		5,140,257	5,140,257
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		5,114,786	5,114,786
Convertible preferred stock, outstanding (in shares)		5,114,786	5,114,786
Series H
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)		4,209,693	4,209,693
Convertible preferred stock, par value (in shares)		$ 0.0001	$ 0.0001
Convertible preferred stock, issued (in shares)		2,075,216	2,075,216
Convertible preferred stock, outstanding (in shares)		2,075,216	2,075,216
Subsequent event
Class of Stock [Line Items]
Common stock, authorized (in shares)	500,000,000
Common stock, par value (in dollars per share)	$ 0.0001
IPO | Subsequent event
Class of Stock [Line Items]
Convertible preferred stock, authorized (in shares)	500,000
Convertible preferred stock, par value (in shares)	$ 0.0001
Number of common stock shares converted	41,562,111
Number of shares issued for each share of convertible preferred stock converted (in shares)	1