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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Assets    
Trading securities $ 0 $ 286,396
Derivative instruments, at fair value 241,952 33,053
Fair Value, Measurements, Recurring    
Assets    
Trading securities   286,396
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)    
Assets    
Trading securities   0
Derivative instruments, at fair value 0 0
Liabilities    
Derivative instruments, at fair value 0 0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)    
Assets    
Trading securities   286,396
Derivative instruments, at fair value 241,952 33,053
Liabilities    
Derivative instruments, at fair value 139,089 257,943
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)    
Assets    
Trading securities   0
Derivative instruments, at fair value 0 0
Liabilities    
Derivative instruments, at fair value 0 0
Commodity Contract | Fair Value, Measurements, Recurring    
Assets    
Derivative instruments, at fair value 241,952 33,053
Liabilities    
Derivative instruments, at fair value $ 139,089 $ 257,943