Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)

v3.8.0.1
Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Assets:    
Derivative contracts $ 1,043 $ 6,434
Restoration plan assets 5,576 5,594
Total 6,619 12,028
Liabilities:    
Derivative contracts 448  
Total 448  
Recurring | Quoted Prices in Active Markets (Level 1)    
Assets:    
Derivative contracts 0 0
Restoration plan assets 5,576 5,594
Total 5,576 5,594
Liabilities:    
Derivative contracts 0  
Total 0  
Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Derivative contracts 1,043 6,434
Restoration plan assets 0 0
Total 1,043 6,434
Liabilities:    
Derivative contracts 448  
Total 448  
Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Derivative contracts 0 0
Restoration plan assets 0 0
Total 0 $ 0
Liabilities:    
Derivative contracts 0  
Total $ 0