Fair Value Measurements (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financial Assets:
Total financial assets	$ 613,065	$ 717,596
Money market deposits
Financial Assets:
Total financial assets	24,619	28,990
U.S. Treasury securities
Financial Assets:
Total financial assets	495,538	597,744
Corporate bonds and notes
Financial Assets:
Total financial assets	16,248	12,328
Municipal securities
Financial Assets:
Total financial assets	76,660	78,534
Level 1
Financial Assets:
Total financial assets	520,157	626,734
Level 1 | Money market deposits
Financial Assets:
Total financial assets	24,619	28,990
Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	495,538	597,744
Level 2
Financial Assets:
Total financial assets	92,908	90,862
Level 2 | Convertible Notes
Liabilities:
Fair value of financial liabilities	792,900
Level 2 | Corporate bonds and notes
Financial Assets:
Total financial assets	16,248	12,328
Level 2 | Municipal securities
Financial Assets:
Total financial assets	$ 76,660	$ 78,534