Fair Value Measurements - Schedule of Financial Instruments and the Related Fair Value Hierarchy of the Valuation Techniques Utilized (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	$ 515,476
Total Financial Liabilities	220,475	$ 134,839
Marketable Securities [Member] | US Government Agency [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	19,982
Marketable Securities [Member] | US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	495,494
Development Derivative Liability [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	220,475	134,839
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	515,476
Level 1 [Member] | Marketable Securities [Member] | US Government Agency [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	19,982
Level 1 [Member] | Marketable Securities [Member] | US Government Obligations [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Assets	495,494
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	220,475	134,839
Level 3 [Member] | Development Derivative Liability [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total Financial Liabilities	$ 220,475	$ 134,839