Stock-Based Compensation - Additional Information (Detail) - USD ($)					1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Jul. 15, 2015	Jul. 14, 2015	Dec. 31, 2014	Feb. 28, 2015	Dec. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance	7,746,692			22,154,290		22,154,290	7,746,692		7,746,692		22,154,290
Proceeds from early exercise of stock options of unvested portion									$ 13,000
Performance-Based Stock Option Grant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional stock-based compensation related to stock options							$ 100,000		$ 100,000
Performance-Based Stock Option Grant [Member] | Executive officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted options to purchase shares of common stock											217,305
2008 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			365,535
Number of shares held by employees subject to repurchase	39,493			103,252		103,252	39,493		39,493		103,252
2008 Plan [Member] | Executive officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options early exercised					13,422	103,252
Proceeds from early exercise of stock options of unvested portion	$ 38,000			$ 100,000
2005 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards issued	940
Awards outstanding	940						940		940
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares initially reserved for issuance			3,400,000
Number of common stock shares reserved for issuance			3,765,535
Stock option grants description									The number of shares initially reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company's common stock as of the immediately preceding December 31.
Percentage threshold of outstanding shares increased annually under the plan			4.00%
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance	700,000	700,000					700,000		700,000
Stock option grants description									The number of shares initially reserved for issuance under the ESPP will increase automatically on January 1 for nine years from the first offering date by the number of shares equal to 1% of the total outstanding shares of the Company's common stock as of the immediately preceding December 31.
Percentage threshold of outstanding shares increased annually under the plan		1.00%
Maximum number of common stock shares allowed to issue under employee stock purchase plan		3,400,000
Options to purchase shares of common stock, discount percentage		15.00%
Number of common stock issued									0
2005 Plan, 2008 Plan and 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards issued									1,217,568
Awards outstanding	3,314,159			2,138,096		2,138,096	3,314,159		3,314,159		2,138,096
Weighted-average grant date fair values of options granted							$ 23.60	$ 1.12	$ 10.19	$ 0.96
Aggregate intrinsic value of options exercised							$ 200,000	$ 200,000	$ 200,000	$ 200,000
Total grant date fair value of options vested							700,000	$ 100,000	900,000	$ 200,000
Total unrecognized stock-based compensation related to unvested stock options	$ 11,900,000						$ 11,900,000		$ 11,900,000
Weighted-average period									3 years 3 months 18 days
Maximum number of common stock shares allowed to issue under employee stock purchase plan	3,400,000						3,400,000		3,400,000