Investments - Valuation of Money Market Funds (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	$ 394,256	$ 275,826
Level 1
Debt and Equity Securities, FV-NI [Line Items]
Amortized cost of held-to-maturity securities	4,600
Fair market value of held-to-maturity securities	4,600
U.S. government bonds
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	27,895	17,608
U.S. government bonds | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	27,895	17,608
U.S. government bonds | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	$ 0	0
U.S. government bonds | Fair Value, Recurring | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond		$ 4,700