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Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Fair Value of Financial Instruments
Money Market Funds	$ 73,808	$ 50,312
Marketable Securities	536,073
Equity Securities with Readily Determinable Fair Value	2,253
Total Fair Value	588,126	583,005
Transfers from level 1 to level 2	0		$ 0
Transfers from level 2 to level 1	0		$ 0
Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	49,800	32,009
Corporate Securities
Fair Value of Financial Instruments
Marketable Securities	332,691	281,751
Government Securities
Fair Value of Financial Instruments
Marketable Securities	203,382	269,245
Level 1
Fair Value of Financial Instruments
Equity Securities with Readily Determinable Fair Value	2,253
Total Fair Value	52,053	32,009
Level 1 \| Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	49,800	32,009
Level 2
Fair Value of Financial Instruments
Total Fair Value	536,073	550,996
Level 2 \| Corporate Securities
Fair Value of Financial Instruments
Marketable Securities	332,691	281,751
Level 2 \| Government Securities
Fair Value of Financial Instruments
Marketable Securities	$ 203,382	$ 269,245