Acquired Intangibles - Amortization of Intangible Assets (Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Finite-Lived Intangible Assets [Line Items]
Amortization of acquired intangibles, net	$ 2,096	$ (343)	$ 2,961	$ (7,026)
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Amortization of acquired intangibles, net	(1,224)	(6,007)	(3,687)	(18,631)
Acquired mine permits, net
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	3,320	5,664	6,648	11,605
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	215	122	587	879
Coal supply agreements, net
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible liabilities	$ (1,439)	$ (6,129)	$ (4,274)	$ (19,510)