Stockholders' Equity - Common stock, resale registration statement, and follow-on public offering (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Class Of Stock [Line Items]
Common Stock Shares Authorized	100,000,000		100,000,000
Common stock, par value	$ 0.0025		$ 0.0025
Other offering expenses	$ 117
Reclassified out of accumulated other comprehensive loss for realized gains or losses on available for sale securities	0	$ 0
Accumulated deficit subject to restriction	$ 8,700		$ 8,700
Minimum
Class Of Stock [Line Items]
Accumulated profits	10.00%		10.00%
Employee Stock Options
Class Of Stock [Line Items]
Common stock	8,509,302
Sale of common stock upon the exercise of overallotment option	133,287
Employee Stock Purchase Plan
Class Of Stock [Line Items]
Common stock	627,848