Contract Obligations (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Mt / yr	Dec. 31, 2018 USD ($)
Other Commitments [Line Items]
Contract obligation	$ 241,812	$ 292,694
Non-Market Contracts [Member]
Other Commitments [Line Items]
Number of tons of trans loading, required to pay from agreement | Mt / yr	1
Contract obligation	$ 25,000
Sales Contracts, Fixed Pricing Component [Member]
Other Commitments [Line Items]
Amortization for the liability	10,000
Non-Market Coal Supply Agreement [Member]
Other Commitments [Line Items]
Non-market portion of the coal leases	27,300
Amortization of liability	33,900	$ 28,300
Amortization for the liability	$ 307,000