Annual report pursuant to Section 13 and 15(d)

Financial Instruments - Fair Value (Details)

v3.8.0.1
Financial Instruments - Fair Value (Details) - Derivatives designated as cash flow hedges: - Cash Flow Hedging - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Asset derivatives:    
Total asset derivatives $ 1,043 $ 6,434
Liability derivatives:    
Total liability derivatives 448 0
Natural gas swaps | Current assets    
Asset derivatives:    
Total asset derivatives 0 573
Natural gas swaps | Other long-term assets    
Asset derivatives:    
Total asset derivatives 0 58
Natural gas swaps | Current liabilities    
Liability derivatives:    
Total liability derivatives 318 0
Natural gas swaps | Other long-term liabilities    
Liability derivatives:    
Total liability derivatives 130 0
Interest rate caps | Current assets    
Asset derivatives:    
Total asset derivatives 44 0
Interest rate caps | Other long-term assets    
Asset derivatives:    
Total asset derivatives $ 999 $ 5,803