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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative [Line Items]
Derivative gain (loss) reclassified from accumulated OCI to current earnings:	$ 270	$ 4	$ 753	$ (337)
Other Comprehensive Income (Loss)
Derivative [Line Items]
Derivative gain (loss) recognized into OCI:	(6,093)	603	(9,608)	2,067
Other Comprehensive Income (Loss) | Natural Gas Contracts
Derivative [Line Items]
Derivative gain (loss) recognized into OCI:	(1,106)	123	(1,143)	334
Other Comprehensive Income (Loss) | Interest Rate Swaps
Derivative [Line Items]
Derivative gain (loss) recognized into OCI:	(4,987)	480	(8,465)	1,733
Cost of Sales | Natural Gas Contracts
Derivative [Line Items]
Derivative gain (loss) reclassified from accumulated OCI to current earnings:	(65)	(36)	63	(234)
Interest Expense | Interest Rate Swaps
Derivative [Line Items]
Derivative gain (loss) reclassified from accumulated OCI to current earnings:	$ 335	$ 40	$ 690	$ (103)