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Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Fair Value of Financial Instruments
Money Market Funds	$ 109,534	$ 50,312
Marketable Securities	475,259
Equity Securities with Readily Determinable Fair Value	2,611
Total Fair Value	581,101	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	103,231	32,009
Corporate Securities
Fair Value of Financial Instruments
Marketable Securities	267,542	281,751
Government Securities
Fair Value of Financial Instruments
Marketable Securities	207,717	269,245
Level 1
Fair Value of Financial Instruments
Equity Securities with Readily Determinable Fair Value	2,611
Total Fair Value	105,842	32,009
Level 1 \| Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	103,231	32,009
Level 2
Fair Value of Financial Instruments
Total Fair Value	475,259	550,996
Level 2 \| Corporate Securities
Fair Value of Financial Instruments
Marketable Securities	267,542	281,751
Level 2 \| Government Securities
Fair Value of Financial Instruments
Marketable Securities	$ 207,717	$ 269,245