Income Taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Federal and state research and development tax credits		$ 9,686	$ 10,558
Accumulated deficit		(1,652,869)	(1,330,545)
Estimated tax benefit upon enactment of Tax Act	$ 2,185
Change in deferred tax assets resulting from enactment of Tax Act	87,473
Estimated tax benefit due to changes in deferred tax liabilities upon enactment of Tax Act	1,730
Estimated current tax benefit upon enactment of Tax Act	455
Provisional transition tax amount	$ 0
Foreign Subsidiaries
Operating Loss Carryforwards [Line Items]
Accumulated deficit		14,900
Domestic
Operating Loss Carryforwards [Line Items]
Net operating losses inherited via acquisition		42,100
Operating loss carryforwards		568,800
Deferred tax assets, capital loss carryforwards		111,600
Federal and state research and development tax credits		9,600
Domestic | GenVec, Inc.
Operating Loss Carryforwards [Line Items]
Net operating losses inherited via acquisition		13,376
Foreign
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		72,800
BELGIUM
Operating Loss Carryforwards [Line Items]
Additional deferred tax liabilities recorded due to enacted foreign tax changes			$ 2,307
Generated after 2017 | Domestic
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		$ 316,100