Fair Value Measurements - Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets:
Available-for-sale securities	$ 16,529	$ 56,929
Short-term investments	16,529	56,929
Municipal bonds and notes
Assets:
Available-for-sale securities		2,986
Zero coupon bonds
Assets:
Available-for-sale securities	13,997	33,919
Corporate bonds
Assets:
Available-for-sale securities	2,002	19,506
Certificates of deposit
Assets:
Available-for-sale securities	530	518
Fair value measured on recurring basis \| Municipal bonds and notes
Assets:
Available-for-sale securities		2,986
Fair value measured on recurring basis \| Zero coupon bonds
Assets:
Available-for-sale securities	13,997	33,919
Fair value measured on recurring basis \| Corporate bonds
Assets:
Available-for-sale securities	2,002	19,506
Fair value measured on recurring basis \| Certificates of deposit
Assets:
Short-term investments	530	518
Fair value measured on recurring basis \| Commodity swap contracts
Assets:
Derivative assets	9,743	3,270
Liabilities:
Derivative liabilities		164
Fair value measured on recurring basis \| Embedded derivatives
Assets:
Derivative assets	94	723
Liabilities:
Derivative liabilities	1,924	81
Fair value measured on recurring basis \| Warrants
Liabilities:
Derivative liabilities		40
Fair value measured on recurring basis \| Level 2 \| Municipal bonds and notes
Assets:
Available-for-sale securities		2,986
Fair value measured on recurring basis \| Level 2 \| Zero coupon bonds
Assets:
Available-for-sale securities	13,997	33,919
Fair value measured on recurring basis \| Level 2 \| Corporate bonds
Assets:
Available-for-sale securities	2,002	19,506
Fair value measured on recurring basis \| Level 2 \| Certificates of deposit
Assets:
Short-term investments	530	518
Fair value measured on recurring basis \| Level 3 \| Commodity swap contracts
Assets:
Derivative assets	9,743	3,270
Liabilities:
Derivative liabilities		164
Fair value measured on recurring basis \| Level 3 \| Embedded derivatives
Assets:
Derivative assets	94	723
Liabilities:
Derivative liabilities	$ 1,924	81
Fair value measured on recurring basis \| Level 3 \| Warrants
Liabilities:
Derivative liabilities		$ 40

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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), classified as current.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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