Quarterly report pursuant to Section 13 or 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
Sep. 30, 2017
Dec. 31, 2016
Asset derivatives:    
Total asset derivatives $ 1,098 $ 6,434
Liability derivatives:    
Total liability derivatives 58 0
Current assets | Natural gas swaps    
Asset derivatives:    
Total asset derivatives 0 573
Current assets | Interest rate caps    
Asset derivatives:    
Total asset derivatives 9 0
Other long-term assets | Natural gas swaps    
Asset derivatives:    
Total asset derivatives 7 58
Other long-term assets | Interest rate caps    
Asset derivatives:    
Total asset derivatives 1,082 5,803
Accrued liabilities | Natural gas swaps    
Liability derivatives:    
Total liability derivatives $ 58 $ 0