Parent Company Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash and cash equivalents	$ 101,600	$ 69,308	$ 35,701	$ 54,148
Accounts receivable from affiliates	19,919	7,404
Property, plant and equipment, net	120,068	115,106
Other non-current assets	23,063	23,089
Total assets	389,552	668,021	655,408
LIABILITIES AND STOCKHOLDERS’ EQUITY
Current liabilities	53,976	221,828
Deferred compensation	12,939	13,578
Other long-term liabilities	12,336	9,737
Stockholders’ equity	219,448	220,293
Total liabilities and equity	389,552	668,021
Parent Company
ASSETS
Cash and cash equivalents	94,646	57,917	$ 22,506	$ 53,415
Accounts receivable from affiliates	9,189	0
Other current assets	1,714	2,518
Investment in subsidiaries	154,514	185,559
Property, plant and equipment, net	310	935
Other non-current assets	9,550	13,870
Total assets	269,923	260,799
LIABILITIES AND STOCKHOLDERS’ EQUITY
Current liabilities	7,627	7,055
Current intercompany accounts payable, net	11,858	1,406
Other long-term liabilities	201	367
Stockholders’ equity	219,448	220,293
Total liabilities and equity	269,923	260,799
Restricted investments	1,800
Unrestricted investment	300
Parent Company | HBB
ASSETS
Investment in subsidiaries	0	44,057
Parent Company | KC
ASSETS
Investment in subsidiaries	0	21,394
Parent Company | NACoal
ASSETS
Investment in subsidiaries	141,174	105,645
Parent Company | Other, primarily Bellaire
ASSETS
Investment in subsidiaries	13,340	14,463
Parent Company | Bellaire
LIABILITIES AND STOCKHOLDERS’ EQUITY
Note payable to Bellaire	$ 17,850	$ 18,100