Accounting for Derivative Instruments - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Commodity Contract
Offsetting Liabilities [Line Items]
Gross Liabilities	$ (30,710)	$ (1,059)
Gross Amounts Offset	19,752	311
Net Liabilities	(10,958)	(748)
Cash Collateral Offset	300	0
Net Amount Presented	(10,658)	(748)
Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(22,311)	(754)
Gross Amounts Offset	15,064	74
Net Liabilities	(7,247)	(680)
Cash Collateral Offset	300	0
Net Amount Presented	(6,947)	(680)
Trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(128)	(92)
Gross Amounts Offset	24	5
Net Liabilities	(104)	(87)
Cash Collateral Offset	0	0
Net Amount Presented	(104)	(87)
Not Designated as Hedging Instrument | Non-trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(22,183)	(662)
Gross Amounts Offset	15,040	69
Net Liabilities	(7,143)	(593)
Cash Collateral Offset	300	0
Net Amount Presented	(6,843)	(593)
Not Designated as Hedging Instrument | Trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	0
Gross Amounts Offset	0
Net Liabilities	0
Cash Collateral Offset	0
Net Amount Presented	0
Not Designated as Hedging Instrument | Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(8,399)	(305)
Gross Amounts Offset	4,688	237
Net Liabilities	(3,711)	(68)
Cash Collateral Offset	0	0
Net Amount Presented	(3,711)	(68)
Not Designated as Hedging Instrument | Non-trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(8,399)	(305)
Gross Amounts Offset	4,688	237
Net Liabilities	(3,711)	(68)
Cash Collateral Offset	0	0
Net Amount Presented	$ (3,711)	$ (68)