Income Taxes (Reconciliation of the Federal Statutory and Effective Income Tax Rate) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income (loss) from continuing operations before income tax provision (benefit)	$ 29,102	$ (6,693)	$ (7,237)
Statutory taxes (benefit) at 35.0%	10,186	(2,343)	(2,533)
State and local income taxes	493	(1,676)	(1,332)
Valuation allowances	(1,453)	2,432	2,480
Non-deductible expenses	224	1,334	424
Percentage depletion	(6,253)	(6,373)	(8,406)
R&D and other federal credits	301	278	(896)
Tax settlements	74	(3,161)	551
Provisional effect of the TCJA	(3,132)	0	0
Other, net	199	(140)	202
Income tax provision (benefit)	$ 639	$ (9,649)	$ (9,510)
Effective income tax rate from continuing operations	2.20%	144.20%	131.40%
Statutory tax rate	35.00%	35.00%	35.00%