Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended	70 Months Ended
Dec. 31, 2014	Dec. 31, 2016	Sep. 30, 2017	Dec. 31, 2015	Mar. 31, 2012
Commitments and Contingencies [Abstract]
Extended lease term	6 years
Deferred rent liability		$ 50	$ 48	$ 48	$ 52
Aggregate consideration paid to NU for agreement obligations		1,534	3,247
Accrued Northwestern University License Agreements fee (Note 11)		$ 1,500	$ 0	$ 0

Carrying value as of the balance sheet date of obligations incurred through that date and payable for contractual rent under lease arrangements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15)(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15)(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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