Fair Value Measurement - Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	$ 145,803	$ 149,680
Mutual and money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	101,696	104,672
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	23,591	22,155
Government securities - U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	17,353	18,999
Other fixed-income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	3,163	3,854
Fair Value Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	101,696	104,672
Fair Value Measurements, Recurring | Level 1 | Mutual and money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	101,696	104,672
Fair Value Measurements, Recurring | Level 1 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 1 | Government securities - U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 1 | Other fixed-income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	44,107	45,008
Fair Value Measurements, Recurring | Level 2 | Mutual and money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 2 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	23,591	22,155
Fair Value Measurements, Recurring | Level 2 | Government securities - U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	17,353	18,999
Fair Value Measurements, Recurring | Level 2 | Other fixed-income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	3,163	3,854
Fair Value Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Fair Value Measurements, Recurring | Level 3 | Mutual and money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 3 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 3 | Government securities - U.S.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	0	0
Fair Value Measurements, Recurring | Level 3 | Other fixed-income securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mutual and money market funds	$ 0	$ 0