Fair Value Measurements - Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Available-for-sale securities	$ 56,929	$ 65,646
Short-term investments	56,929	65,646
Municipal bonds and notes
Assets:
Available-for-sale securities	2,986	9,191
Zero coupon bonds
Assets:
Available-for-sale securities	33,919	29,795
Corporate bonds
Assets:
Available-for-sale securities	19,506	26,153
Certificates of deposit
Assets:
Available-for-sale securities	518	507
Fair value measured on recurring basis \| Municipal bonds and notes
Assets:
Available-for-sale securities	2,986	9,191
Fair value measured on recurring basis \| Zero coupon bonds
Assets:
Available-for-sale securities	33,919	29,795
Fair value measured on recurring basis \| Corporate bonds
Assets:
Available-for-sale securities	19,506	26,153
Fair value measured on recurring basis \| Certificates of deposit
Assets:
Short-term investments	518	507
Fair value measured on recurring basis \| Commodity swap contracts
Assets:
Derivative assets	3,270	10,332
Liabilities:
Derivative liabilities	164
Fair value measured on recurring basis \| Embedded derivatives
Assets:
Derivative assets	723
Liabilities:
Derivative liabilities	81
Fair value measured on recurring basis \| Warrants
Liabilities:
Derivative liabilities	40	1,079
Fair value measured on recurring basis \| Level 2 \| Municipal bonds and notes
Assets:
Available-for-sale securities	2,986	9,191
Fair value measured on recurring basis \| Level 2 \| Zero coupon bonds
Assets:
Available-for-sale securities	33,919	29,795
Fair value measured on recurring basis \| Level 2 \| Corporate bonds
Assets:
Available-for-sale securities	19,506	26,153
Fair value measured on recurring basis \| Level 2 \| Certificates of deposit
Assets:
Short-term investments	518	507
Fair value measured on recurring basis \| Level 3 \| Commodity swap contracts
Assets:
Derivative assets	3,270	10,332
Liabilities:
Derivative liabilities	164
Fair value measured on recurring basis \| Level 3 \| Embedded derivatives
Assets:
Derivative assets	723
Liabilities:
Derivative liabilities	81
Fair value measured on recurring basis \| Level 3 \| Warrants
Liabilities:
Derivative liabilities	$ 40	$ 1,079

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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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