Derivatives Summary of gains (losses) recognized in Statement of Operations and OCI (Details) - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated OCI to current earnings	$ 711	$ (1,780)
Not Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Gain (loss) recognized in current earnings	0	(1,036)
Other Comprehensive Income (Loss) \| Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Effective portion of derivative gain (loss) recognized in OCI	(3,242)	(286)
Natural Gas Contracts \| Other Comprehensive Income (Loss) \| Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Effective portion of derivative gain (loss) recognized in OCI	1,194	(1,019)
Natural Gas Contracts \| Cost of Sales \| Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated OCI to current earnings	426	(45)
Natural Gas Contracts \| Other Income (Expense) \| Not Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Gain (loss) recognized in current earnings	0	(1,036)
Interest Rate Swaps \| Other Comprehensive Income (Loss) \| Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Effective portion of derivative gain (loss) recognized in OCI	(4,436)	733
Interest Rate Swaps \| Interest Expense \| Designated as Hedging Instrument
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Effective portion of derivative gain (loss) reclassified from accumulated OCI to current earnings	$ 285	$ (1,735)

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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