INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Net deferred tax asset	$ 110,633	$ 15,274
Research and development credit	15,509	3,207
Increase (decrease) in deferred tax asset valuation allowance	(95,400)	(3,500)
Unrecognized tax benefits	21,700
Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	403,911	36,400
Operating loss carryforwards, not subject to expiration	25,100	2,400
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	350,557	31,000
Vical
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	89,300
Research and development credit	9,300	9,300
Vical | Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	253,100
Tax credit carryforwards	24,600
Research and development credit	10,800
Vical | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	291,500
Federal research and development credits and orphan drug credit | Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	$ 30,300	$ 3,200