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Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017	Nov. 30, 2017
Debt Instrument [Line Items]
Long-term debt, net	$ 685,855		$ 826,490
Long term debt, gross	697,365
Less: unamortized debt issuance costs	10,323		16,409
Less amounts due in one year	32,053		117,954
Long-Term Debt	653,802		708,536
Current portion of finance lease	18,219
Current portion of finance lease			16,858
Loans Payable \| Term Loan B
Debt Instrument [Line Items]
Long-term debt, net	271,751		389,747
Long term debt, gross	272,938		396,000
Less: unamortized debt issuance costs	$ 1,187		$ 6,253
Weighted average interest rate	6.30%		8.53%
Loans Payable \| MEDCO Revenue Bonds in Series due September 2025 at 5.75%
Debt Instrument [Line Items]
Long term debt, gross	$ 102,865		$ 102,865
Stated interest rate	5.75%
Loans Payable \| Term Loan A
Debt Instrument [Line Items]
Long term debt, gross	$ 88,750		$ 73,750
Weighted average interest rate	5.55%		6.78%
Loans Payable \| Advance Royalty Commitments
Debt Instrument [Line Items]
Long term debt, gross	$ 1,895		$ 2,261
Weighted average interest rate	10.78%		8.57%
Senior Notes \| Senior Secured Second Lien Notes due 2025
Debt Instrument [Line Items]
Long term debt, gross	$ 221,628		$ 274,276
Stated interest rate	11.00%	11.00%		11.00%	11.00%	11.00%
Secured Debt \| Other Asset-Backed Financing Arrangements
Debt Instrument [Line Items]
Long term debt, gross	$ 9,289		$ 0