Quarterly report pursuant to Section 13 or 15(d)

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

v3.19.1
Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets:    
Derivative contracts (Note 13) $ 27,994 $ 20,768
Restoration plan assets 4,422 4,244
Total 32,416 25,012
Liabilities:    
Derivative contracts (Note 13) 2,892 2,026
Quoted Prices in Active Markets (Level 1)    
Assets:    
Derivative contracts (Note 13) 0 0
Restoration plan assets 4,422 4,244
Total 4,422 4,244
Liabilities:    
Derivative contracts (Note 13) 0 0
Significant Other Observable Inputs (Level 2)    
Assets:    
Derivative contracts (Note 13) 27,994 20,768
Restoration plan assets 0 0
Total 27,994 20,768
Liabilities:    
Derivative contracts (Note 13) 2,892 2,026
Significant Unobservable Inputs (Level 3)    
Assets:    
Derivative contracts (Note 13) 0 0
Restoration plan assets 0 0
Total 0 0
Liabilities:    
Derivative contracts (Note 13) $ 0 $ 0