Investments - Valuation of Money Market Funds (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	$ 410,233
Level 1
Debt and Equity Securities, FV-NI [Line Items]
Cash equivalent money market funds	314,676
Level 2
Debt and Equity Securities, FV-NI [Line Items]
Cash equivalent money market funds	37,294
U.S. government bonds
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	14,014
U.S. government bonds | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	14,014
U.S. government bonds | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond	$ 0
U.S. government bonds | Fair Value, Recurring
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond		$ 6,300
U.S. government bonds | Fair Value, Recurring | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investment government bond		6,348
Money Market Funds | Fair Value, Recurring | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Cash equivalent money market funds		384,474
Money Market Funds | Fair Value, Recurring | Level 1 | Revision of Prior Period, Adjustment
Debt and Equity Securities, FV-NI [Line Items]
Cash equivalent money market funds		$ 286,700