Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Assets:
Cash and cash equivalents	$ 246,992,000	$ 228,698,000
Available for sale securities	0	39,992,000
Liabilities:
Derivative liability	1,930,000	2,420,000
U.S. government debt securities
Assets:
Available for sale securities	0	39,992,000
Level 3
Assets:
Total assets fair value disclosure	0	0
Fair value measurements recurring
Assets:
Cash and cash equivalents	246,992,000	228,698,000
Total assets fair value disclosure	246,992,000	268,690,000
Liabilities:
Derivative liability	1,930,000	2,420,000
Total liabilities fair value disclosure	1,930,000	2,420,000
Fair value measurements recurring | U.S. government debt securities
Assets:
Available for sale securities		39,992,000
Fair value measurements recurring | Level 1
Assets:
Cash and cash equivalents	246,992,000	228,698,000
Total assets fair value disclosure	246,992,000	228,698,000
Liabilities:
Derivative liability	0	0
Total liabilities fair value disclosure	0	0
Fair value measurements recurring | Level 1 | U.S. government debt securities
Assets:
Available for sale securities		0
Fair value measurements recurring | Level 2
Assets:
Cash and cash equivalents	0	0
Total assets fair value disclosure	0	39,992,000
Liabilities:
Derivative liability	0	0
Total liabilities fair value disclosure	0	0
Fair value measurements recurring | Level 2 | U.S. government debt securities
Assets:
Available for sale securities		39,992,000
Fair value measurements recurring | Level 3
Assets:
Cash and cash equivalents	0	0
Total assets fair value disclosure	0	0
Liabilities:
Derivative liability	1,930,000	2,420,000
Total liabilities fair value disclosure	$ 1,930,000	2,420,000
Fair value measurements recurring | Level 3 | U.S. government debt securities
Assets:
Available for sale securities		$ 0