Annual report pursuant to Section 13 and 15(d)

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

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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Assets:    
Derivative contracts $ 20,768 $ 1,043
Restoration plan assets 4,244 5,576
Total 25,012 6,619
Liabilities:    
Derivative contracts 2,026 448
Recurring | Quoted Prices in Active Markets (Level 1)    
Assets:    
Derivative contracts 0 0
Restoration plan assets 4,244 5,576
Total 4,244 5,576
Liabilities:    
Derivative contracts 0 0
Recurring | Significant Other Observable Inputs (Level 2)    
Assets:    
Derivative contracts 20,768 1,043
Restoration plan assets 0 0
Total 20,768 1,043
Liabilities:    
Derivative contracts 2,026 448
Recurring | Significant Unobservable Inputs (Level 3)    
Assets:    
Derivative contracts 0 0
Restoration plan assets 0 0
Total 0 0
Liabilities:    
Derivative contracts $ 0 $ 0