Fair Value Measurements - Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Available-for-sale securities	$ 29,830	$ 56,929
Short-term investments	29,830	56,929
Municipal bonds and notes
Assets:
Available-for-sale securities		2,986
Zero coupon bonds
Assets:
Available-for-sale securities	26,863	33,919
Corporate bonds
Assets:
Available-for-sale securities	2,449	19,506
Certificates of deposit
Assets:
Available-for-sale securities	518	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	26,863	33,919
Fair value measured on recurring basis \| Corporate bonds
Assets:
Available-for-sale securities	2,449	19,506
Fair value measured on recurring basis \| Certificates of deposit
Assets:
Short-term investments	518	518
Fair value measured on recurring basis \| Commodity swap contracts
Assets:
Derivative assets	12,545	3,270
Liabilities:
Derivative liabilities		164
Fair value measured on recurring basis \| Embedded derivatives
Assets:
Derivative assets	143	723
Liabilities:
Derivative liabilities	3,308	81
Fair value measured on recurring basis \| Warrants
Liabilities:
Derivative liabilities	445	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	26,863	33,919
Fair value measured on recurring basis \| Level 2 \| Corporate bonds
Assets:
Available-for-sale securities	2,449	19,506
Fair value measured on recurring basis \| Level 2 \| Certificates of deposit
Assets:
Short-term investments	518	518
Fair value measured on recurring basis \| Level 3 \| Commodity swap contracts
Assets:
Derivative assets	12,545	3,270
Liabilities:
Derivative liabilities		164
Fair value measured on recurring basis \| Level 3 \| Embedded derivatives
Assets:
Derivative assets	143	723
Liabilities:
Derivative liabilities	3,308	81
Fair value measured on recurring basis \| Level 3 \| Warrants
Liabilities:
Derivative liabilities	$ 445	$ 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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Fair value, after the effects of master netting arrangements, of a financial asset or other 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 assets not subject to a master netting arrangement and not elected to be offset.

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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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