Equity Method Investments and Membership Interests in Joint Ventures (Schedule of Equity Method Investments) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance		$ 105,588	$ 105,588	$ 104,676
Sale of investment in affiliates				(1,879)
Advances to (distributions from) affiliates, net			(2,686)	(384)
Equity in comprehensive income (loss)			3,348	3,175
Contribution of Viper Mine			1,700
Impairment of equity investment			(36,167)
Ending Balance			71,783	105,588
Knight Hawk
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance		90,211	90,211	90,021
Sale of investment in affiliates				(1,879)
Advances to (distributions from) affiliates, net			(4,235)	(5,884)
Equity in comprehensive income (loss)			4,576	7,953
Contribution of Viper Mine			1,700
Impairment of equity investment	$ (36,200)	(36,200)	(36,167)
Ending Balance			56,085	90,211
DTA
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance		$ 15,377	15,377	14,655
Advances to (distributions from) affiliates, net			1,549	5,500
Equity in comprehensive income (loss)			(1,228)	(4,778)
Ending Balance			$ 15,698	$ 15,377

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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Amount of gain (loss) on sale or disposal of an equity method investment.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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Amount of increase (decrease) in the investments in and advances to affiliates.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

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