Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Assets:
Total	$ 320,282,000	$ 366,617,000
Level 1
Assets:
Total	138,458,000	286,061,000
Level 2
Assets:
Total	181,824,000	80,556,000
Level 3
Assets:
Total	0	0
Money Market Funds
Assets:
Total	57,964,000	204,632,000
Money Market Funds | Level 1
Assets:
Total	57,964,000	204,632,000
Commercial Paper
Assets:
Total	59,361,000	42,208,000
Commercial Paper | Level 2
Assets:
Total	59,361,000	42,208,000
Corporate Debt Securities
Assets:
Total	71,419,000	25,716,000
Corporate Debt Securities | Level 2
Assets:
Total	71,419,000	25,716,000
Equity Securities
Assets:
Total	35,280,000	41,644,000
Equity Securities | Level 1
Assets:
Total	35,280,000	41,644,000
Asset-backed Securities
Assets:
Total	37,792,000	12,632,000
Asset-backed Securities | Level 2
Assets:
Total	37,792,000	12,632,000
U.S. Government Securities
Assets:
Total	45,214,000	39,785,000
U.S. Government Securities | Level 1
Assets:
Total	45,214,000	$ 39,785,000
Supranational Debt Securities
Assets:
Total	13,252,000
Supranational Debt Securities | Level 2
Assets:
Total	$ 13,252,000