Financial Instruments - Effect on Other Comprehensive Income (Details) - Net loss (gain) from hedging activities - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax
AOCI derivative gain at beginning of year	$ 4,881	$ 0
Amount of gain (loss) reclassified from accumulated OCI into income (effective portion)
AOCI derivative gain (loss) at end of year	(917)	4,881
Interest rate caps
Amount of gain (loss) recognized in OCI on derivatives (effective portion)
Amount of gain (loss) recognized in OCI on derivatives (effective portion)	(4,760)	4,250
Interest rate caps \| Interest expense
Amount of gain (loss) reclassified from accumulated OCI into income (effective portion)
Amount of gain (loss) reclassified from accumulated OCI into income (effective portion)	40	0
Natural gas swaps
Amount of gain (loss) recognized in OCI on derivatives (effective portion)
Amount of gain (loss) recognized in OCI on derivatives (effective portion)	(1,300)	(802)
Natural gas swaps \| Cost of goods sold
Amount of gain (loss) reclassified from accumulated OCI into income (effective portion)
Amount of gain (loss) reclassified from accumulated OCI into income (effective portion)	$ 222	$ 1,433

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, including portion attributable to noncontrolling interest. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

+ References

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

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The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

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