Acquired Intangibles (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Assets	$ 115,628	$ 125,145
Liabilities	(3,183)	(6,018)
Net Total	112,445	119,127
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Assets	546	917
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Assets	115,082	124,228
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Liabilities	(3,183)	(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	(2,637)	(5,101)
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Net Total	$ 115,082	$ 124,228