Summary of Significant Accounting Polices - Reverse Stock Split thru Fair Value of Financial Instruments (Details)		1 Months Ended	12 Months Ended
	Oct. 31, 2013 USD ($) shares	Jun. 30, 2015	Dec. 31, 2015 USD ($) shares	Dec. 31, 2013 USD ($)	Dec. 31, 2014 USD ($) shares	Dec. 31, 2012 USD ($)
Common Stock
Reverse stock split ratio (as a percent)		0.077821
Initial public offering.
Issuance of preferred stock (in shares) | shares			0		0
Proceeds from issuance of preferred stock			$ 12,331,000	$ 41,422,000
Cash and Cash Equivalents
Cash			776,000		$ 1,162,000
Money market fund			62,004,000		13,506,000
Total Cash and Cash Equivalents			62,780,000	$ 30,474,000	$ 14,668,000	$ 5,434,000
Fair Value of Financial Instruments
Transfers of assets or liabilities between levels			$ 0
Series B Convertible Preferred Stock
Initial public offering.
Issuance of preferred stock (in shares) | shares	9,029,549
Proceeds from issuance of preferred stock	$ 0.001