Income Taxes (Reconciliation of the Federal Statutory and Effective Income Tax Rate) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income (loss) before income tax provision (benefit)	$ 24,799	$ (76,573)	$ 55,720
Statutory taxes (benefit) at 35.0%	8,679	(26,801)	19,502
State and local income taxes	(439)	(7,112)	136
Valuation allowance	3,525	5,742	(12)
Non-deductible expenses	787	632	1,081
Percentage depletion	(8,406)	(8,572)	(8,057)
R&D and other federal credits	(1,854)	(1,397)	(1,173)
Other, net	(332)	322	520
Tax settlements	855	(1,269)	(727)
Income tax provision	$ 2,815	$ (38,455)	$ 11,270
Effective income tax rate	11.40%	50.20%	20.20%
Statutory tax rate	35.00%	35.00%	35.00%
Cumulative unremitted earnings of foreign subsidiaries	$ 8,600
Cumulative deferred tax liabilities of unremitted foreign earnings	$ 296	$ 223