Stockholders' equity - Common stock, resale registration statement, and follow-on public offering (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Apr. 30, 2015
Class Of Stock [Line Items]
Common Stock Shares Authorized	100,000,000			100,000,000		100,000,000
Common stock, par value	$ 0.0025			$ 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
Reclassified out of accumulated other comprehensive income for realized gains or losses on available for sale securities	$ 0.0	$ 0.0		$ 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 value							$ 0.0025
Minimum [Member]
Class Of Stock [Line Items]
Accumulated profits						10.00%
Employee Stock Options
Class Of Stock [Line Items]
Common stock	7,220,893			7,220,893
Sale of common stock upon the exercise of overallotment option				962,366
Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock	776,613			776,613