Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Assets:
Cash and cash equivalents	$ 228,698,000	$ 147,449,000
Liabilities:
Derivative liability	2,420,000	1,650,000
Level 3
Assets:
Total assets fair value disclosure	0	0
Fair value measurements recurring
Assets:
Cash and cash equivalents	228,698,000	147,449,000
Total assets fair value disclosure	268,690,000	147,694,000
Liabilities:
Derivative liability	2,420,000	1,650,000
Total liabilities fair value disclosure	2,420,000	1,650,000
Fair value measurements recurring | U.S. government debt securities
Assets:
U.S. government debt securities	39,992,000
Fair value measurements recurring | Certificates of deposit
Assets:
U.S. government debt securities		245,000
Fair value measurements recurring | Level 1
Assets:
Cash and cash equivalents	228,698,000	147,449,000
Total assets fair value disclosure	228,698,000	147,449,000
Liabilities:
Derivative liability	0	0
Total liabilities fair value disclosure	0	0
Fair value measurements recurring | Level 1 | U.S. government debt securities
Assets:
U.S. government debt securities	0
Fair value measurements recurring | Level 1 | Certificates of deposit
Assets:
U.S. government debt securities		0
Fair value measurements recurring | Level 2
Assets:
Cash and cash equivalents	0	0
Total assets fair value disclosure	39,992,000	245,000
Liabilities:
Derivative liability	0	0
Total liabilities fair value disclosure	0	0
Fair value measurements recurring | Level 2 | U.S. government debt securities
Assets:
U.S. government debt securities	39,992,000
Fair value measurements recurring | Level 2 | Certificates of deposit
Assets:
U.S. government debt securities		245,000
Fair value measurements recurring | Level 3
Assets:
Cash and cash equivalents	0	0
Total assets fair value disclosure	0	0
Liabilities:
Derivative liability	2,420,000	1,650,000
Total liabilities fair value disclosure	2,420,000	1,650,000
Fair value measurements recurring | Level 3 | U.S. government debt securities
Assets:
U.S. government debt securities	$ 0
Fair value measurements recurring | Level 3 | Certificates of deposit
Assets:
U.S. government debt securities		$ 0