Mergers and Acquisitions - GenVec - Additional Information (Details) - GenVec, Inc. $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
Jun. 30, 2017 shares	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)
Business Acquisition [Line Items]
Percentage of outstanding common stock acquired	100.00%	100.00%	100.00%
Business combination, consideration paid, shares issued \| shares	684,240
Percent of collaboration payments	50.00%
Expected useful life of intangible asset	11 years
Period subsequent to acquisition during which a portion of collaboration payments received will be paid to former stockholders of acquired entity	3 years
Business combination, acquisition related costs			$ 510
Selling, general and administrative
Business Acquisition [Line Items]
Business combination, acquisition related costs		$ 260	$ 498

Number of shares of equity interests issued or issuable to acquire entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

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Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The period subsequent to the acquisition during which a portion of collaboration payments received from previously existing collaborations will be paid to former stockholders of the acquired entity.

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Percent of collaboration payments received pursuant to a collaboration agreement for upfront fees, milestones and royalties which will be paid to another party.

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