Commitments (Narrative) (Details)		12 Months Ended
	Aug. 11, 2017 USD ($)	Dec. 31, 2017 USD ($) state item shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Aug. 01, 2016 shares
Commitments [Line Items]
Number of primary operating locations | state		3
Payment of surrender cost	$ 1,100,000
Proceeds from rent abatement		$ 1,100,000
Number of former employees | item		2
Amount payable upon consummation of IPO		$ 1,250,000
Common stock issued | shares		78,643,954	45,078,666
Employment agrrement liability		$ 1,200,000	$ 1,200,000
Severance expense		2,900,000	2,900,000
Future Discretionary Clinical Developments and Regulatory Agency Actions [Member]
Commitments [Line Items]
Additional contingent milestone payments		400,400,000
New York [Member]
Commitments [Line Items]
Rental expense		$ 5,700,000	6,400,000	$ 6,200,000
Base rent annual percentage increase		3.50%
Pennsylvania [Member]
Commitments [Line Items]
Expiration date		Sep. 30, 2019
Renewal option		5 years
Rental expense		$ 600,000	600,000	600,000
Massachusetts [Member]
Commitments [Line Items]
Expiration date		Apr. 01, 2023
Rental expense		$ 300,000	300,000	$ 0
Secured Letter of Credit [Member] | New York [Member]
Commitments [Line Items]
Secured letter of credit		2,000,000	$ 2,000,000
Secured Letter of Credit [Member] | Massachusetts [Member]
Commitments [Line Items]
Secured letter of credit		$ 91,000
Former Employees [Member]
Commitments [Line Items]
Common stock issued | shares					208,334