Summary of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, T in Millions	1 Months Ended			12 Months Ended
	Jan. 31, 2019 USD ($) $ / shares	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Dec. 31, 2018 USD ($) T	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Revenue, Major Customer [Line Items]
Allowance for doubtful accounts				$ 0	$ 0
Surety bonds				26,000,000
DD&A				44,167,000	$ 38,495,000
Subsequent Event [Member]
Revenue, Major Customer [Line Items]
Right-of-use asset						$ 500,000
Operating lease liabilities						$ 500,000
Dividends, date declared	2019-01
Dividends payable, amount per share | $ / shares	$ 0.04
Dividends payable, date of record	Jan. 31, 2019
Dividends payable, date to be paid	Feb. 15, 2019
Gain on sale of investment	$ 2,500,000
Accounting Standards Update 2014-09 [Member]
Revenue, Major Customer [Line Items]
Revenue				500,000
Accounting Standards Update 2016-01 [Member]
Revenue, Major Customer [Line Items]
Cumulative-effect adjustment from accumulated other comprehensive income to retained earnings				$ 1,100,000
Accounting Standards Update 2018 02 [Member]
Revenue, Major Customer [Line Items]
Cumulative-effect adjustment from accumulated other comprehensive income to retained earnings			$ 192,000
Sales Revenue, Goods, Net [Member] | Five Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of revenue					92.00%
Sales Revenue, Goods, Net [Member] | Four Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of revenue				82.00%
Accounts Receivable [Member] | Five Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of revenue				77.00%
Accounts Receivable [Member] | Four Customers [Member]
Revenue, Major Customer [Line Items]
Percentage of revenue					83.00%
Coal Supply Commitment [Member]
Revenue, Major Customer [Line Items]
Coal supply commitment | T				30.5
Year that supply commitments end				2024
Priced coal supply commitment | T				20.7
Minimum [Member]
Revenue, Major Customer [Line Items]
Discount rate				5.00%
Estimated useful lives				P3Y
Maximum [Member]
Revenue, Major Customer [Line Items]
Discount rate				10.00%
Estimated useful lives				P25Y
Savoy [Member]
Revenue, Major Customer [Line Items]
Equity method investment ownership percentage		30.60%
Proceeds from the sale of investment		$ 8,000,000
Proceeds from the sale of investment, net commissions paid				$ 7,500,000
Sunrise Energy [Member]
Revenue, Major Customer [Line Items]
Equity method investment ownership percentage				50.00%