Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Transfers between Levels 1 and 2
Fair value, assets transfers between Level 1 and Level II	$ 0
Recurring | Available-for-sale securities
Financial Assets:
Total financial assets	717,596	$ 401,542
Recurring | Money market deposits
Financial Assets:
Total financial assets	28,990	22,477
Recurring | U.S. Treasury securities
Financial Assets:
Total financial assets	597,744	293,157
Recurring | Corporate bonds and notes
Financial Assets:
Total financial assets	12,328
Recurring | Municipal securities
Financial Assets:
Total financial assets	78,534	85,908
Recurring | Level 1 | Available-for-sale securities
Financial Assets:
Total financial assets	626,734	315,634
Recurring | Level 1 | Money market deposits
Financial Assets:
Total financial assets	28,990	22,477
Recurring | Level 1 | U.S. Treasury securities
Financial Assets:
Total financial assets	597,744	293,157
Recurring | Level 2 | Available-for-sale securities
Financial Assets:
Total financial assets	90,862	85,908
Recurring | Level 2 | Corporate bonds and notes
Financial Assets:
Total financial assets	12,328
Recurring | Level 2 | Municipal securities
Financial Assets:
Total financial assets	$ 78,534	$ 85,908