Convertible Redeemable Preferred Stock - Additional Information (Details)			1 Months Ended				9 Months Ended
	Oct. 03, 2019 shares	Feb. 23, 2018 USD ($) $ / shares shares	Oct. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) Director $ / shares shares	Sep. 30, 2018 USD ($)
Redeemable Convertible Preferred Stock [Line Items]
Redeemable convertible preferred stock				40,618,706		37,695,912	40,618,706
Redeemable convertible preferred stock,Per share | $ / shares				$ 0.001		$ 0.001	$ 0.001
Proceeds from issuance of redeemable convertible preferred stock | $		$ 282,253,000					$ 167,000,000	$ 282,253,000
Receivables from redeemable convertible preferred stock | $						$ 12,000,000
Number of directors | Director							8
Dividends declared or paid | $							$ 0
IPO | Subsequent Event
Redeemable Convertible Preferred Stock [Line Items]
Proceeds from public offering | $			$ 136,266,000
Common Stock
Redeemable Convertible Preferred Stock [Line Items]
Preferred stock, liquidation preference | $				$ 468,995,000		$ 312,253,000	$ 468,995,000
Common Stock | IPO | Subsequent Event
Redeemable Convertible Preferred Stock [Line Items]
Number of outstanding preferred stock converted into aggregate shares of common stock	40,618,706
Maximum
Redeemable Convertible Preferred Stock [Line Items]
Shares issued, price per share | $ / shares				$ 17.60			$ 17.60
Minimum [Member]
Redeemable Convertible Preferred Stock [Line Items]
Proceeds from public offering | $							$ 75,000,000
Series A-1 Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Redeemable convertible preferred stock				14,225,324		14,225,324	14,225,324
Redeemable convertible preferred stock,Per share | $ / shares		$ 0.001		$ 0.001		$ 0.001	$ 0.001
Convertible redeemable preferred stock, shares issued		14,225,324		14,225,324		14,225,324	14,225,324
Convertible redeemable preferred stock purchase price per share | $ / shares		$ 10.00
Preferred stock holder entitled to elect number of directors | Director							2
Series A-2 Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Redeemable convertible preferred stock				17,000,000		17,000,000	17,000,000
Redeemable convertible preferred stock,Per share | $ / shares		$ 0.001		$ 0.001		$ 0.001	$ 0.001
Convertible redeemable preferred stock, shares issued		14,000,000		17,000,000		17,000,000	17,000,000
Convertible redeemable preferred stock purchase price per share | $ / shares		$ 10.00
Proceeds from issuance of redeemable convertible preferred stock | $						$ 30,000,000
Convertible redeemable preferred stock issuance of additional shares						3,000,000
Convertible redeemable preferred stock additional price per shares | $ / shares		$ 10.00
Receivables from redeemable convertible preferred stock | $						$ 12,000,000
Percentage of redeemable convertible preferred stock converted to common stock redeemed							90.00%
Preferred stock holder entitled to elect number of directors | Director							4
Series A-2 Preferred Stock | Maximum
Redeemable Convertible Preferred Stock [Line Items]
Convertible redeemable preferred stock issuance of additional shares		3,000,000
Series A-3 Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Redeemable convertible preferred stock				4,705,882		6,470,588	4,705,882
Redeemable convertible preferred stock,Per share | $ / shares				$ 0.001		$ 0.001	$ 0.001
Convertible redeemable preferred stock, shares issued				4,705,882		0	4,705,882
Convertible redeemable preferred stock purchase price per share | $ / shares		$ 17.00		$ 17.00
Proceeds from issuance of redeemable convertible preferred stock | $				$ 80,000,000
Series A-3 Preferred Stock | Maximum
Redeemable Convertible Preferred Stock [Line Items]
Convertible redeemable preferred stock, shares issued		6,470,588
Series B Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Redeemable convertible preferred stock				4,687,500		0	4,687,500
Redeemable convertible preferred stock,Per share | $ / shares				$ 0.001	$ 0.001	$ 0.001	$ 0.001
Convertible redeemable preferred stock, shares issued				4,687,500	4,687,500	0	4,687,500
Convertible redeemable preferred stock purchase price per share | $ / shares					$ 16.00
Proceeds from issuance of redeemable convertible preferred stock | $					$ 75,000,000
Preferred stock holder entitled to elect number of directors | Director							1
Preferred stock, dividend per share | $ / shares							$ 1.28
Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Preferred stock holder entitled to elect number of directors | Director							7
Series A Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Affirmative vote of outstanding shares percentage							75.00%
Series A-2 and A-3 Redeemable Convertible Preferred Stock
Redeemable Convertible Preferred Stock [Line Items]
Affirmative vote of outstanding shares percentage							75.00%