Supplemental Financial Information - Prepaid expenses and other assets and Other assets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Prepaid expenses and other assets:
Federal and state income tax benefits	$ 11,416	$ 0
Prepaid federal and state income taxes	4,215	4,228
Prepaid expenses	1,987	1,708
Other	1,341	1,896
Other current assets	18,959	7,832
Other long-term assets, net:
Right of use assets, operating leases, net	2,529	5,073
Spare parts, net	3,559	3,453
Mine development costs, net	4,139	7,084
Mine reclamation asset, net	1,323	2,451
Highview Investment	552	552
Other long-term assets	1,779	1,718
Total	$ 13,881	$ 20,331