Stockholders' equity - Common stock, resale registration statement, and follow-on public offering (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Apr. 30, 2015
Class Of Stock [Line Items]
Common Stock Shares Authorized	100,000,000			100,000,000
Common Stock Par Or Stated Value Per Share	$ 0.0025			$ 0.0025
Common stock, par value	$ 0.0025			$ 0.0025
Issuance of common stock from public stock offering, net of discount and offering costs (in shares)		6,866,689
Sale of common stock upon the exercise of overallotment option		895,655
Public offering price		$ 7.25
Consideration raised on transaction		$ 45.6
Underwriting discounts		3.0
Other offering expenses		$ 1.2
Other Comprehensive Income Loss Reclassification Adjustment From AOCI For Sale Of Securities Before Ta	$ 0.0		$ 0.0
Accumulated deficit subject to restriction				$ 7.9
Resale Registration Statement [Member]
Class Of Stock [Line Items]
Common Stock Shares Authorized					4,972,905
Common Stock Par Or Stated Value Per Share					$ 0.0025
Common stock, par value					$ 0.0025
Minimum [Member]
Class Of Stock [Line Items]
Accumulated profits				10.00%
Employee Stock Options
Class Of Stock [Line Items]
Common stock	7,995,640
Sale of common stock upon the exercise of overallotment option	349,954
Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock	978,503