Quarterly report pursuant to Section 13 or 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) - Recurring - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Assets:    
Derivative contracts $ 10,223 $ 1,043
Restoration plan assets 5,217 5,576
Total 15,440 6,619
Liabilities:    
Derivative contracts 16,026 448
Total 16,026 448
Quoted Prices in Active Markets (Level 1)    
Assets:    
Derivative contracts 0 0
Restoration plan assets 5,217 5,576
Total 5,217 5,576
Liabilities:    
Derivative contracts 0 0
Total 0 0
Significant Other Observable Inputs (Level 2)    
Assets:    
Derivative contracts 10,223 1,043
Restoration plan assets 0 0
Total 10,223 1,043
Liabilities:    
Derivative contracts 16,026 448
Total 16,026 448
Significant Unobservable Inputs (Level 3)    
Assets:    
Derivative contracts 0 0
Restoration plan assets 0 0
Total 0 0
Liabilities:    
Derivative contracts 0 0
Total $ 0 $ 0