Redeemable Convertible Preferred Stock and Stockholders' Equity/(Deficit) - Summary of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2017	Dec. 31, 2015
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	166,266,114	0
Redeemable convertible preferred stock, issued (in shares)	55,422,002	0
Redeemable convertible preferred stock outstanding (in shares)	55,422,002	0	55,422,002
Aggregate Liquidation Preference	$ 167,488	$ 0
Proceeds, Net of Issuance Costs	$ 164,173
Series A-1
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	4,378,284
Redeemable convertible preferred stock, issued (in shares)	1,459,427
Redeemable convertible preferred stock outstanding (in shares)	1,459,427
Aggregate Liquidation Preference	$ 500
Proceeds, Net of Issuance Costs	$ 462
Series A-2
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	109,552
Redeemable convertible preferred stock, issued (in shares)	36,517
Redeemable convertible preferred stock outstanding (in shares)	36,517
Aggregate Liquidation Preference	$ 11
Proceeds, Net of Issuance Costs	$ 11
Series A-3
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	9,099,610
Redeemable convertible preferred stock, issued (in shares)	3,033,202
Redeemable convertible preferred stock outstanding (in shares)	3,033,202
Aggregate Liquidation Preference	$ 259
Proceeds, Net of Issuance Costs	$ 241
Series B
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	36,338,577
Redeemable convertible preferred stock, issued (in shares)	12,112,853
Redeemable convertible preferred stock outstanding (in shares)	12,112,853
Aggregate Liquidation Preference	$ 7,998
Proceeds, Net of Issuance Costs	$ 7,952
Series C
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	33,388,982
Redeemable convertible preferred stock, issued (in shares)	11,129,656
Redeemable convertible preferred stock outstanding (in shares)	11,129,656
Aggregate Liquidation Preference	$ 12,000
Proceeds, Net of Issuance Costs	$ 11,950
Series D
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	28,574,005
Redeemable convertible preferred stock, issued (in shares)	9,524,665
Redeemable convertible preferred stock outstanding (in shares)	9,524,665
Aggregate Liquidation Preference	$ 10,269
Proceeds, Net of Issuance Costs	$ 10,201
Series E
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	12,041,148
Redeemable convertible preferred stock, issued (in shares)	4,013,712
Redeemable convertible preferred stock outstanding (in shares)	4,013,712
Aggregate Liquidation Preference	$ 14,924
Proceeds, Net of Issuance Costs	$ 14,841
Series F
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	20,808,580
Redeemable convertible preferred stock, issued (in shares)	6,936,186
Redeemable convertible preferred stock outstanding (in shares)	6,936,186
Aggregate Liquidation Preference	$ 50,536
Proceeds, Net of Issuance Costs	$ 50,453
Series G
Temporary Equity [Line Items]
Redeemable convertible preferred stock, authorized (in shares)	21,527,376
Redeemable convertible preferred stock, issued (in shares)	7,175,784
Redeemable convertible preferred stock outstanding (in shares)	7,175,784
Aggregate Liquidation Preference	$ 70,991
Proceeds, Net of Issuance Costs	$ 68,062