Summary of Significant Accounting Policies - Net Loss Per Share thru Segment Information (Details)	12 Months Ended
	Dec. 31, 2016 segment shares	Dec. 31, 2015 shares	Dec. 31, 2014 shares
Antidilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss Per Share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	2,294,735	1,782,959	9,286,250
Segment Information
Number of operating segments | segment	1
Convertible preferred stock
Antidilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss Per Share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)			8,012,988
Options to purchase Common Stock
Antidilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss Per Share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	2,270,169	1,723,554	1,226,140
Warrants for the purchase of Common Stock
Antidilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss Per Share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	24,566	59,405	34,839
Convertible preferred stock warrants | Preferred stock warrants
Antidilutive Common Stock Equivalents Excluded from Computation of Diluted Net Loss Per Share
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)			12,283