Summary of Significant Accounting Policies - Net Loss per Share (Details) - shares	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Anti-dilutive securities
Total shares excluded from diluted net loss per share applicable to common stockholders									3,732,643	2,846,668	2,548,320
Weighted-average number of common shares used in net loss per share applicable to common stockholders- basic and diluted	33,694,756	33,653,479	32,892,712	30,335,164	26,316,550	26,259,216	25,584,147	23,393,448	32,656,237	25,406,845	12,696,414
Stock options
Anti-dilutive securities
Total shares excluded from diluted net loss per share applicable to common stockholders									3,732,643	2,846,668	2,543,435
Unvested Restricted Stock
Anti-dilutive securities
Total shares excluded from diluted net loss per share applicable to common stockholders											4,885