Acquired Intangibles (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Assets	$ 100,350	$ 125,145
Liabilities	(875)	(6,018)
Net Total	99,475	119,127
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Assets	185	917
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Assets	100,165	124,228
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Liabilities	(875)	(6,018)
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Liabilities	0	0
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Net Total	(690)	(5,101)
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Net Total	$ 100,165	$ 124,228