Other Long-Term Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Other Liabilities Disclosure [Abstract]
Black lung obligations	$ 25,297	$ 25,206
Property, plant, and equipment obligation	32,727	0
Other	3,070	5,510
Total other long-term liabilities	61,094	30,716
Capital lease, noncash and not included in the Condensed Statements of Cash Flows	$ 2,100	$ 2,100