Acquired Intangibles (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Assets	$ 103,439	$ 125,145
Liabilities	(1,745)	(6,018)
Net Total	101,694	119,127
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Assets	330	917
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Assets	103,109	124,228
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Liabilities	(1,745)	(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	(1,415)	(5,101)
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Net Total	$ 103,109	$ 124,228