Schedule I - Condensed Financial Information of the Parent (Condensed Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash and cash equivalents	$ 85,257	$ 101,600	$ 69,308
Accounts receivable from affiliates	7,999	19,919
Property, plant and equipment, net	124,554	120,068
Other non-current assets	24,412	23,063
Total assets	376,991	389,552
LIABILITIES AND EQUITY
Current liabilities	42,248	53,976
Deferred compensation	12,939	12,939
Other long-term liabilities	15,581	12,336
Stockholders’ equity	250,704	219,448
Total liabilities and equity	376,991	389,552
Parent Company
ASSETS
Cash and cash equivalents	84,819	94,646	$ 57,917
Accounts receivable from affiliates	2,418	9,189
Current intercompany accounts receivable, net	868	0
Other current assets	4,508	1,714
Investment in subsidiaries:	185,653	154,514
Property, plant and equipment, net	241	310
Other non-current assets	7,851	9,550
Total assets	286,358	269,923
LIABILITIES AND EQUITY
Current liabilities	5,148	7,627
Current intercompany accounts payable, net	0	11,858
Other long-term liabilities	267	201
Stockholders’ equity	250,704	219,448
Total liabilities and equity	286,358	269,923
Parent Company | NACoal
ASSETS
Investment in subsidiaries:	173,020	141,174
Parent Company | Other, primarily Bellaire
ASSETS
Investment in subsidiaries:	12,633	13,340
Parent Company | Bellaire
LIABILITIES AND EQUITY
Note payable to Bellaire	$ 17,300	$ 17,850