For the periods presented, the following significant instruments are accounted for on a fair value basis:

During 2020, we entered into certain forward natural gas contracts (“natural gas contracts”), which are accounted for on a mark-to-market basis. We are utilizing these natural gas contracts as economic hedges for risk management purposes, but these contracts are not designated as hedging instruments. At December 31, 2020, our natural gas contracts included 7.3 million MMBtu of natural gas and extend through December 2021 (there were none at December 31, 2019). The valuations of the natural gas contracts are classified as Level 2. At December 31, 2020, the valuation inputs included the contractual weighted-average cost of $2.65 per MMBtu and the weighted-average market value of $2.49 per MMBtu.

For 2020, we recognized a $1.6 million loss (none for 2019 or 2018), classified as cost of sales, which amount includes an unrealized loss of $1.2 million attributed to natural gas contracts still held at the reporting date.

As discussed in Note 10, certain embedded features (“embedded derivative”) relating to the redemption of the Series E Redeemable Preferred, which includes certain contingent redemption features and the participation rights value have been bifurcated from the Series E Redeemable Preferred and recorded as a liability.

At December 31, 2020 and 2019, we had estimated that the contingent redemption features had fair value since we had assessed that it was probable that a portion of the shares of this preferred stock would have been redeemed prior to October 25, 2023. For certain other embedded features, we had estimated no fair value.

The fair value of the embedded derivative included using discounted cash flow models and primarily based on the difference in the present value of estimated future cash flows with no redemptions prior to October 25, 2023 compared to certain redemptions deemed probable during the same period and applying the effective dividend rate of the Series E Redeemable Preferred.

At December 31, 2020 and 2019, the fair value of the embedded derivative included the valuation of the participation rights, which was based on the equivalent of 303,646 shares of our common stock at $3.39 and $4.20 per share, respectively.

The valuations of the embedded derivative are classified as Level 3. This derivative is valued using market information, management’s redemption assumptions, the underlying number of shares as defined in the terms of the Series E Redeemable Preferred, and the market price of our common stock.

For 2020, 2019 and 2018, we recognized unrealized gains of approximately $0.1 million, $0.5 million and $1.2 million, respectively, due to the change in fair value of the embedded derivative. These unrealized gains are included in non-operating other income and expense.

The following details our assets and liabilities that are measured at fair value on a recurring basis at December 31, 2020 and 2019:

			Fair Value Measurements at December 31, 2020 Using
Description	Total Fair Value at December 31, 2020		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3) ⁽¹⁾		Total Fair Value at December 31, 2019
	(In Thousands)
Assets - Supplies, prepaid items and other:
Natural gas contracts	$	80	$	—	$	80	$	—	$	—
Total	$	80	$	—	$	80	$	—	$	—

Liabilities - Current and noncurrent accrued and other liabilities:
Natural gas contracts	$	1,285	$	—	$	1,285	$	—	$	—
Embedded derivative	$	1,029	$	—	$	—	$	1,029	$	1,084
Total	$	2,314	$	—	$	1,285	$	1,029	$	1,084