Annual report pursuant to Section 13 and 15(d)

Financial Instruments - Effect on Other Comprehensive Income (Details)

v3.8.0.1
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