Stockholders' Equity - Common stock, resale registration statement, and follow-on public offering (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, par value (USD per share)	$ 0.0025	$ 0.0025
Public offering price (USD per share)				$ 7.25
Proceeds from issuance of common stock				$ 45,600
Underwriting discounts				3,000
Other offering expenses				$ 1,200
Reclassified out of accumulated other comprehensive loss for realized gains or losses on available for sale securities	$ 0	$ 0	$ 0
Retained earnings subject to restriction	$ 9,000	$ 8,800
Resale Registration Statement
Class of Stock [Line Items]
Common stock, shares authorized (in shares)			4,972,905	4,972,905
Common stock, par value (USD per share)			$ 0.0025	$ 0.0025
Minimum
Class of Stock [Line Items]
Accumulated profits	10.00%
Employee stock options
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	7,486,096
Employee stock purchase plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	474,462
Common stock | Follow-on public offering
Class of Stock [Line Items]
Issuance of common stock from public stock offering, net of discount and offering costs (in shares)				6,866,689
Common stock | Over-Allotment Option
Class of Stock [Line Items]
Issuance of common stock from public stock offering, net of discount and offering costs (in shares)				895,655