Investments in Affiliates (Details Textual) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Jan. 31, 2018
Carbon California [Member]
Investments in Affiliates (Textual)
Percentage of interests		17.81%
Carbon Appalachia [Member]
Investments in Affiliates (Textual)
Ownership interest, percentage	100.00%
Class A Units [Member] \| Carbon Appalachia [Member]
Investments in Affiliates (Textual)
Percentage of interests	10.00%
Investment in cash and unevaluated property	$ 6.9
Revolving credit facility, description	We acquired all of Old Ironsides Class A Units of Carbon Appalachia for approximately $58.1 million, subject to certain closing adjustments. We paid $ 33.0 million in cash and issued the Old Ironsides Notes in the aggregate original principal amount of approximately $25.1 million to Old Ironsides.
Earned net gain	$ 1.0
Class B Units [Member]
Investments in Affiliates (Textual)
Ownership interest, percentage	14.70%
Class B Units [Member] \| Carbon Appalachia [Member]
Investments in Affiliates (Textual)
Earned net gain	$ 1.0
Class C Units [Member]
Investments in Affiliates (Textual)
Ownership interest, percentage	14.70%
Class C Units [Member] \| Carbon Appalachia [Member]
Investments in Affiliates (Textual)
Percentage of interests	10.00%
Investment in cash and unevaluated property	$ 6.9
Earned net gain	$ 1.0

No definition available.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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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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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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Description of the terms of a credit facility arrangement. Terms typically include interest rate, collateral required, guarantees required, repayment requirements, and restrictions on use of assets and activities of the entity.

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Stated as a percentage, the estimated or actual value of the asset at the end of its useful life or when it is no longer serviceable (cannot be used for its original purpose) divided by its [historical] capitalized cost.

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