Annual report pursuant to Section 13 and 15(d)

Derivative Financial Instruments, Schedule of Derivative Instruments, Gain (Loss) in Statements of Financial Performance (Details)

v3.6.0.2
Derivative Financial Instruments, Schedule of Derivative Instruments, Gain (Loss) in Statements of Financial Performance (Details) - Not Designated as Hedging Instrument - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Derivative Instruments, Gain (Loss)      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 5,818 $ 10,965 $ 1,282
Cost of Sales | Forward Contracts | Commodity Contracts      
Derivative Instruments, Gain (Loss)      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 5,451 (6,302) (18,950)
Cost of Sales | Swaps | Commodity Contracts      
Derivative Instruments, Gain (Loss)      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 367 $ 17,267 $ 20,232