Long-Term Debt and Finance Lease Obligations - Schedule of Long-Term Debt and Finance Lease Obligations (Parenthetical) (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Term Loan due 2022
Debt Instrument [Line Items]
Debt instrument maturity year	2022	2022
Second Lien Notes due 2023
Debt Instrument [Line Items]
Debt instrument maturity year	2023	2023
Debt instrument interest percentage	11.50%
5.78% longwall financing arrangement
Debt Instrument [Line Items]
Debt instrument interest percentage	5.78%	5.78%
5.555% longwall financing arrangement
Debt Instrument [Line Items]
Debt instrument interest percentage	5.555%	5.555%
Revolving Credit Facility ($170.0 Million capacity)
Debt Instrument [Line Items]
Maximum borrowings capacity under credit facility	$ 170,000,000

Debt instrument maturity year.

No definition available.

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Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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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.

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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- Details

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- Details

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- Details

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- Details

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