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Accounting for Derivative Instruments - Offsetting Assets (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Commodity Contract    
Offsetting Assets [Line Items]    
Gross Assets $ 4,473 $ 28,145
Gross Amounts Offset (2,085) (16,718)
Net Assets 2,388 11,427
Cash Collateral Offset 0 0
Net Amount Presented 2,388 11,427
Commodity Contract, Current    
Offsetting Assets [Line Items]    
Gross Assets 4,413 20,271
Gross Amounts Offset (2,025) (11,927)
Net Assets 2,388 8,344
Cash Collateral Offset 0 0
Net Amount Presented 2,388 8,344
Trading Commodity Contract, Current    
Offsetting Assets [Line Items]    
Gross Assets (23) 614
Gross Amounts Offset 31 (83)
Net Assets 8 531
Cash Collateral Offset 0 0
Net Amount Presented 8 531
Not Designated as Hedging Instrument | Non-trading Commodity Contract, Current    
Offsetting Assets [Line Items]    
Gross Assets 4,436 19,657
Gross Amounts Offset (2,056) (11,844)
Net Assets 2,380 7,813
Cash Collateral Offset 0 0
Net Amount Presented 2,380 7,813
Not Designated as Hedging Instrument | Commodity Contract, Noncurrent    
Offsetting Assets [Line Items]    
Gross Assets 60 7,874
Gross Amounts Offset (60) (4,791)
Net Assets 0 3,083
Cash Collateral Offset 0 0
Net Amount Presented 0 3,083
Not Designated as Hedging Instrument | Non-trading Commodity Contract, Noncurrent    
Offsetting Assets [Line Items]    
Gross Assets 60 7,874
Gross Amounts Offset (60) (4,791)
Net Assets 0 3,083
Cash Collateral Offset 0 0
Net Amount Presented $ 0 $ 3,083