Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Sep. 30, 2015	Jul. 15, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Total common stock reserved for future issuance	7,746,692		22,154,290
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	700,000	700,000
Outstanding Preferred Stock Warrants [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			976,492
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			252,817
Series B Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			1,947,354
Series B-1 Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			1,392,300
Series C Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			2,487,770
Series D Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance			11,409,360
Outstanding And Issued Stock Options [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	3,314,159		2,138,096
Shares Reserved For Future Option Grants [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	3,732,533		1,550,101