Subsequent Events (Details)				3 Months Ended
	Apr. 28, 2017 USD ($) ft² item	Apr. 04, 2017 USD ($)	Dec. 13, 2016 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Follow-On Public Offering
Proceeds from issuance of common stock				$ 902,000	$ 29,000
Follow-on Offering | Common Stock
Follow-On Public Offering
Proceeds from issuance of common stock			$ 134,500,000
Subsequent Event | Office and Laboratory Space in Cambridge Massachusetts, 45 Sidney Street
Lease Agreement
Area leased (in square feet) | ft²	99,833
Lease term	146 months
Number of extensions | item	2
Extension period of lease term	5 years
Base annual rent, initial	$ 641,000
Increase in operating leases base annual rent percentage	3.00%
Additional annual rent increase percentage	3.00%
Monthly base rent increase period	14 months
Base annual rent, maximum	$ 887,000
Allowance for leasehold improvements	14,200,000
Security deposit	3,500,000
Reduction in security deposit	$ 1,000,000
Subsequent Event | Follow-on Offering | Common Stock
Follow-On Public Offering
Proceeds from issuance of common stock		$ 215,600,000