Sales Contracts (Schedule of Sales Contracts) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2015	Dec. 31, 2014
Successor [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Prepaid royalties	$ 0
Acquired fair value, Assets	97,196
Acquired fair value, Liabilities	31,742
Accumulated amortization, Asset	(25,625)
Accumulated amortization, Liabilities	(24,829)
Total, Assets	71,571
Total, Liabilities	6,913
Net total, Liabilities	64,658
Coal Supply Agreement, Liabilities	5,114
Successor [Member] | Other Current Assets [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	59,702
Successor [Member] | Accrued Liabilities [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	5,114
Successor [Member] | Other Assets [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	11,869
Successor [Member] | Other Noncurrent Liabilities [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	$ 1,799
Predecessor [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Prepaid royalties		$ 3,406	$ 41,990	$ 15,356
Acquired fair value, Assets			131,299
Acquired fair value, Liabilities			166,697
Accumulated amortization, Asset			(130,839)
Accumulated amortization, Liabilities			(151,354)
Total, Assets			460
Total, Liabilities			15,343
Net total, Liabilities			(14,883)
Coal Supply Agreement, Liabilities			3,852
Predecessor [Member] | Other Current Assets [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets			460
Predecessor [Member] | Accrued Liabilities [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities			3,852
Predecessor [Member] | Other Assets [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets			0
Predecessor [Member] | Other Noncurrent Liabilities [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities			$ 11,491