Stockholders' Equity - Issuance of Common Stock and Warrants (Details)		1 Months Ended	6 Months Ended
	May 29, 2018 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Jul. 01, 2018 USD ($) $ / shares	Jul. 02, 2017 USD ($)	Dec. 31, 2017 $ / shares	Jun. 30, 2017 $ / shares shares	Mar. 16, 2017 USD ($) offering
Class Of Stock [Line Items]
Shelf registration, number of offerings | offering							1
Shelf registration, maximum offering | $							$ 40,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001	$ 0.001		$ 0.001
Number of common stock contain in each unit | shares	1
Number of warrant contain in each unit | shares	0.40
Exercise price of each unit | $ / shares	$ 1.15
Net proceeds from issuance of common stock | $	$ 13,900,000	$ 15,200,000	$ 15,855,000	$ 17,304,000
Underwriting discounts and other offering expenses | $	$ 1,600,000		$ 1,638,000	$ 1,771,000
Exercisable period of warrants	60 months
Strike price of warrants (in dollars per share) | $ / shares	$ 1.38
Fair value assumptions of warrants, expected term	5 years
Underwritten public offering price | $ / shares	$ 1.15	$ 1.50	$ 1.15
June 2012 financing
Class Of Stock [Line Items]
Strike price of warrants (in dollars per share) | $ / shares						$ 2.98
Warrants outstanding (in shares) | shares						2,300,000
Measurement Input, Share Price
Class Of Stock [Line Items]
Warrants and Rights Outstanding, Measurement Input	0.57
Measurement Input, Risk Free Interest Rate
Class Of Stock [Line Items]
Warrants and Rights Outstanding, Measurement Input	2.58
Measurement Input, Price Volatility
Class Of Stock [Line Items]
Warrants and Rights Outstanding, Measurement Input	52.75
Measurement Input, Expected Dividend Rate
Class Of Stock [Line Items]
Warrants and Rights Outstanding, Measurement Input	0
Common stock
Class Of Stock [Line Items]
Aggregate shares of common stock issued | shares	13,500,000	11,300,000
Common stock | Maximum
Class Of Stock [Line Items]
Warrants to purchase aggregate shares of common stock | shares	5,400,000