Derivatives (Summary of gains (losses) recognized in Statement of Operations and AOCI) (Details) - Cash Flow Hedging - Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Derivative gain (loss) reclassified from accumulated OCI to current earnings:	$ 483	$ (341)
Other Comprehensive Income (Loss)
Derivative [Line Items]
Derivative gain (loss) recognized into OCI:	(3,515)	1,464
Other Comprehensive Income (Loss) | Natural Gas Contracts
Derivative [Line Items]
Derivative gain (loss) recognized into OCI:	(37)	211
Other Comprehensive Income (Loss) | Interest Rate Swaps
Derivative [Line Items]
Derivative gain (loss) recognized into OCI:	(3,478)	1,253
Cost of Sales | Natural Gas Contracts
Derivative [Line Items]
Derivative gain (loss) reclassified from accumulated OCI to current earnings:	128	(198)
Interest Expense | Interest Rate Swaps
Derivative [Line Items]
Derivative gain (loss) reclassified from accumulated OCI to current earnings:	$ 355	$ (143)