Stockholders' equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2015	Sep. 30, 2015	Dec. 31, 2014	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
Sale of common stock 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			$ 7.1
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		7,009,303
Sale of common stock upon the exercise of overallotment option		102,711
Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock		678,438