Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying financial statements are presented in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements. In our opinion, the financial statements include all adjustments (consisting of normal recurring accruals) necessary in order to make the financial statements not misleading.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include short-term deposits and highly liquid investments that have original maturities of three months or less when purchased and are stated at cost, which approximates fair value.
Short-Term Investments	Short-Term Investments Instruments with maturities greater than three months, but less than twelve months, are included in short-term investments. The Company purchases United States Treasury ("Treasury") bills with maturities ranging from six to twelve months which are classified as held to maturity and are carried at amortized cost, which approximates fair value. The Company also purchases fixed income securities and certificates of deposits with varying maturities that are classified as available for sale and are carried at fair value. Securities classified as held to maturity are those securities that management has the intent and ability to hold to maturity.
Revenue Recognition	Revenue Recognition Revenue is recognized when performance obligations under the terms of a contract with the Company's customers are satisfied; for all contracts this occurs when control of the promised goods have been transferred to its customers. For coal shipments to domestic customers via rail, control is transferred when the railcar is loaded. For coal shipments to international customers via ocean vessel, control is transferred when the vessel is loaded at the Port of Mobile, Alabama. For natural gas sales, control is transferred when the gas has been transferred to the pipeline. Revenue is disaggregated between coal sales within the Company's mining segment and natural gas sales which is included in all other revenues, as disclosed in Note 13.
Trade Accounts Receivable and Allowance for Credit Losses	Trade Accounts Receivable and Allowance for Credit Losses Trade accounts receivable represent customer obligations that are derived from revenue recognized from contracts with customers. Credit is extended based on an evaluation of the individual customer's financial condition. The Company maintains trade credit insurance on the majority of its customers and the geographic regions of coal shipments to these customers. In some instances, the Company requires letters of credit, cash collateral or prepayments from its customers on or before shipment to mitigate the risk of loss. These efforts have consistently resulted in the Company recognizing no historical credit losses. The Company also has never had to have a claim against its trade credit insurance policy. In order to estimate the allowance for credit losses on trade accounts receivable, the Company utilizes an aging approach in which potential impairment is calculated based on how long a receivable has been outstanding (e.g., current, 1-31, 31-60, etc.). The Company calculates an expected credit loss rate based on the Company’s historical credit loss rate, the risk characteristics of our customers, and the current metallurgical coal and steel market environments. As of March 31, 2020, the estimated allowance for credit losses was immaterial and did not have a material impact on the Company's financial statements.
New Accounting Pronouncements	New Accounting Pronouncements The Company adopted Accounting Standards Update 2016-13, "Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments" as of January 1, 2020 using the modified retrospective approach. The ASU requires the use of an “expected loss” model for instruments measured at amortized cost, in which companies will be required to estimate the lifetime expected credit loss and record an allowance to offset the amortized cost basis, resulting in a net presentation of the amount expected to be collected on the financial asset. The adoption of the new standard did not have a material impact on the Company's financial statements, including accounting policies, processes and systems.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

The accompanying financial statements are presented in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements. In our opinion, the financial statements include all adjustments (consisting of normal recurring accruals) necessary in order to make the financial statements not misleading.

Cash and Cash Equivalents

Cash and cash equivalents include short-term deposits and highly liquid investments that have original maturities of three months or less when purchased and are stated at cost, which approximates fair value.

Short-Term Investments

Instruments with maturities greater than three months, but less than twelve months, are included in short-term investments. The Company purchases United States Treasury ("Treasury") bills with maturities ranging from six to twelve months which are classified as held to maturity and are carried at amortized cost, which approximates fair value. The Company also purchases fixed income securities and certificates of deposits with varying maturities that are classified as available for sale and are carried at fair value. Securities classified as held to maturity are those securities that management has the intent and ability to hold to maturity.

Revenue Recognition

Revenue is recognized when performance obligations under the terms of a contract with the Company's customers are satisfied; for all contracts this occurs when control of the promised goods have been transferred to its customers. For coal shipments to domestic customers via rail, control is transferred when the railcar is loaded. For coal shipments to international customers via ocean vessel, control is transferred when the vessel is loaded at the Port of Mobile, Alabama. For natural gas

sales, control is transferred when the gas has been transferred to the pipeline. Revenue is disaggregated between coal sales within the Company's mining segment and natural gas sales which is included in all other revenues, as disclosed in Note 13.

Trade Accounts Receivable and Allowance for Credit Losses

Trade accounts receivable represent customer obligations that are derived from revenue recognized from contracts with customers. Credit is extended based on an evaluation of the individual customer's financial condition. The Company maintains trade credit insurance on the majority of its customers and the geographic regions of coal shipments to these customers. In some instances, the Company requires letters of credit, cash collateral or prepayments from its customers on or before shipment to mitigate the risk of loss. These efforts have consistently resulted in the Company recognizing no historical credit losses. The Company also has never had to have a claim against its trade credit insurance policy.

In order to estimate the allowance for credit losses on trade accounts receivable, the Company utilizes an aging approach in which potential impairment is calculated based on how long a receivable has been outstanding (e.g., current, 1-31, 31-60, etc.). The Company calculates an expected credit loss rate based on the Company’s historical credit loss rate, the risk characteristics of our customers, and the current metallurgical coal and steel market environments. As of March 31, 2020, the estimated allowance for credit losses was immaterial and did not have a material impact on the Company's financial statements.

New Accounting Pronouncements

The Company adopted Accounting Standards Update 2016-13, "Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments" as of January 1, 2020 using the modified retrospective approach. The ASU requires the use of an “expected loss” model for instruments measured at amortized cost, in which companies will be required to estimate the lifetime expected credit loss and record an allowance to offset the amortized cost basis, resulting in a net presentation of the amount expected to be collected on the financial asset. The adoption of the new standard did not have a material impact on the Company's financial statements, including accounting policies, processes and systems.