Coal supply commitment

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- Definition

Percentage of profits after capital investment and return are met

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Percentage of profits until capital investment and return met

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No definition available.

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- Definition

Priced coal supply commitment

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No definition available.

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- Definition

Proceeds From Sale Of Equity Method Investments Net Of Commissions Paid

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Reduction to depreciation, depletion, and amortization

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No definition available.

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- Definition

Year Supply Commitments End

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No definition available.

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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- Definition

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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-Publisher FASB
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-Section 45
-Paragraph 28
-Subparagraph (b)
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- Definition

Date the declared dividend will be paid, in CCYY-MM-DD format.

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- Definition

The per share amount of a dividend declared, but not paid, as of the financial reporting date.

+ References

+ Details

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- Definition

The month and year (YYYY-MM) that the dividend to be paid was declared.

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- Definition

Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
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- Definition

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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-Name Accounting Standards Codification
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-Paragraph 3
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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

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-SubTopic 10
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-Paragraph 2
-Subparagraph (bbb)
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- Definition

Fair value as of the balance sheet date of interest rate derivative assets, net of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

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- Definition

Number of interest rate derivative instruments held by the entity at the reporting date.

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- Definition

The cash inflow associated with the sale of 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
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-Subparagraph (b)
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- Definition

Describes the periods of time over which an entity anticipates to receive utility from its property, plant and equipment (that is, the periods of time over which an entity allocates the initial cost of its property, plant and equipment).

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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-Name Accounting Standards Codification
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Summary of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, $ in Thousands, T in Millions	1 Months Ended			12 Months Ended
	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Jan. 31, 2018 $ / shares	Dec. 31, 2017 USD ($) security T	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 03, 2016	Nov. 02, 2016
Revenue, Major Customer [Line Items]
Surety bonds				$ 25,000
DD&A				38,495	$ 35,565	$ 43,942
Subsequent Event [Member]
Revenue, Major Customer [Line Items]
Dividends, date declared			2018-01
Dividends payable, amount per share \| $ / shares			$ 0.04
Dividends payable, date of record			Jan. 31, 2018
Dividends payable, date to be paid			Feb. 16, 2018
Interest Rate Swap [Member]
Revenue, Major Customer [Line Items]
Interest swaps, net				$ 500
Number of interest rate swaps \| security				2
Change from Straight Line Method to Units of Production Method [Member]
Revenue, Major Customer [Line Items]
Reduction to depreciation, depletion, and amortization					$ 2,600
Sales Revenue, Goods, Net [Member] \| Five Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of total from largest customers				92.00%	90.00%
Sales Revenue, Goods, Net [Member] \| Four Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of total from largest customers						82.00%
Accounts Receivable [Member] \| Four Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of total from largest customers				83.00%	78.00%
Coal Supply Commitment [Member]
Revenue, Major Customer [Line Items]
Coal supply commitment \| T				26.1
Year that supply commitments end				2024
Priced coal supply commitment \| T				13.5
Minimum [Member]
Revenue, Major Customer [Line Items]
Discount rate				5.00%
Estimated useful lives				3 years
Maximum [Member]
Revenue, Major Customer [Line Items]
Discount rate				10.00%
Estimated useful lives				25 years
Savoy [Member]
Revenue, Major Customer [Line Items]
Equity method investment ownership percentage				30.60%			30.60%	40.80%
Savoy [Member] \| Scenario, Forecast [Member]
Revenue, Major Customer [Line Items]
Proceeds from the sale of investment	$ 8,000
Proceeds from the sale of investment, net commissions paid	$ 7,500
Sunrise Energy [Member]
Revenue, Major Customer [Line Items]
Equity method investment ownership percentage				50.00%
Hourglass Sands [Member] \| Subsequent Event [Member]
Revenue, Major Customer [Line Items]
Initial investment to purchase equity		$ 4,000
Percentage of profits until capital investment and return are met		100.00%
Hourglass Sands [Member] \| Subsequent Event [Member] \| Common Class A [Member]
Revenue, Major Customer [Line Items]
Equity method investment ownership percentage		100.00%
Percentage of profits after capital investment and return are met		90.00%
Hourglass Sands [Member] \| Yorktown Company [Member] \| Subsequent Event [Member]
Revenue, Major Customer [Line Items]
Initial investment to purchase equity		$ 4,000
Carlisle Assets [Member]
Revenue, Major Customer [Line Items]
Reduction to depreciation, depletion, and amortization					$ 4,400