Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
U.S. government agency securities
Fair Value of Financial Instruments
Investments, available-for-sale	$ 100,996	$ 128,312
U.S. Treasury obligations
Fair Value of Financial Instruments
Investments, available-for-sale	274,266	305,710
Money market funds
Fair Value of Financial Instruments
Cash equivalents	341,711	98,946
U.S. Treasury obligations
Fair Value of Financial Instruments
Cash equivalents	29,999	14,992
Recurring
Fair Value of Financial Instruments
Total	746,972	547,960
Recurring | U.S. government agency securities
Fair Value of Financial Instruments
Investments, available-for-sale	100,996	128,312
Recurring | U.S. Treasury obligations
Fair Value of Financial Instruments
Investments, available-for-sale	274,266	305,710
Recurring | Money market funds
Fair Value of Financial Instruments
Cash equivalents	341,711	98,946
Recurring | U.S. Treasury obligations
Fair Value of Financial Instruments
Cash equivalents	29,999	14,992
Recurring | Active Markets (Level 1)
Fair Value of Financial Instruments
Total	645,976	419,648
Recurring | Active Markets (Level 1) | U.S. Treasury obligations
Fair Value of Financial Instruments
Investments, available-for-sale	274,266	305,710
Recurring | Active Markets (Level 1) | Money market funds
Fair Value of Financial Instruments
Cash equivalents	341,711	98,946
Recurring | Active Markets (Level 1) | U.S. Treasury obligations
Fair Value of Financial Instruments
Cash equivalents	29,999	14,992
Recurring | Observable Inputs (Level 2)
Fair Value of Financial Instruments
Total	100,996	128,312
Recurring | Observable Inputs (Level 2) | U.S. government agency securities
Fair Value of Financial Instruments
Investments, available-for-sale	$ 100,996	$ 128,312