Common Stock and Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	9 Months Ended
	Sep. 10, 2015	Apr. 07, 2015	Mar. 06, 2015	Mar. 31, 2014	Sep. 30, 2015
Common Stock
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock issued for warrant exercised				553,274
Common stock, exercise price				$ 1.667
Restricted Common Stock (Founders Shares)
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Vesting period					4 years
Common stock shares subject to repurchase					0
Warrant Issued For Past Services | Research and Development Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense				$ 1,300
Warrant Issued For Past Services | General and Administrative Expense
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense				$ 1,400
Follow-On Public Offering
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share price		$ 29.00
Common stock, shares issued		4,137,931
Gross proceeds from issuance of common stock		$ 120,000
Underwriting discounts and commissions		7,200
Offering expenses		600			$ 601
Proceeds from issuance of common stock, net of offering expense		$ 112,200
Private Placement | Baxalta License Agreement
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Share price	$ 25.63
Common stock, shares issued	390,167
Gross proceeds from issuance of common stock	$ 10,000
InteKrin Therapeutics Inc
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Business acquisition, number of shares issued			358,384
Business acquisition, shares price			$ 27.48
Business acquisition, cash payment			$ 1,000
Business acquisition, value of shares issued			$ 9,800