Acquired Intangibles - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Asset impairment, change in carrying value	$ 33,709,000	$ 0
Amortization of acquired intangibles, net	865,000	(6,683,000)
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	3,328,000	5,941,000
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	372,000	757,000
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible liabilities	(2,835,000)	(13,381,000)
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Amortization of acquired intangibles, net	$ (2,463,000)	$ (12,624,000)