Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019
Assets:
Cash and cash equivalents	$ 245,406,000	$ 147,449,000
Available for sale securities	49,943,000	245,000
Liabilities:
Derivative liability	1,890,000	1,650,000
U.S. government debt securities
Assets:
Available for sale securities	49,943,000
Certificates of deposit
Assets:
Available for sale securities		245,000
Level 3
Assets:
Total assets fair value disclosure	0	0
Fair value measurements recurring
Assets:
Cash and cash equivalents	245,406,000	147,449,000
Total assets fair value disclosure	295,349,000	147,694,000
Liabilities:
Derivative liability	1,890,000	1,650,000
Total liabilities fair value disclosure	1,890,000	1,650,000
Fair value measurements recurring \| U.S. government debt securities
Assets:
Available for sale securities	49,943,000
Fair value measurements recurring \| Certificates of deposit
Assets:
Available for sale securities		245,000
Fair value measurements recurring \| Level 1
Assets:
Cash and cash equivalents	245,406,000	147,449,000
Total assets fair value disclosure	245,406,000	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:
Available for sale securities	0
Fair value measurements recurring \| Level 1 \| Certificates of deposit
Assets:
Available for sale securities		0
Fair value measurements recurring \| Level 2
Assets:
Cash and cash equivalents	0	0
Total assets fair value disclosure	49,943,000	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:
Available for sale securities	49,943,000
Fair value measurements recurring \| Level 2 \| Certificates of deposit
Assets:
Available for sale 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	1,890,000	1,650,000
Total liabilities fair value disclosure	1,890,000	1,650,000
Fair value measurements recurring \| Level 3 \| U.S. government debt securities
Assets:
Available for sale securities	$ 0
Fair value measurements recurring \| Level 3 \| Certificates of deposit
Assets:
Available for sale securities		$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Name Accounting Standards Codification
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Fair value of financial and nonfinancial obligations.

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