Accounting for Derivative Instruments - Offsetting Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commodity Contract
Offsetting Liabilities [Line Items]
Gross Liabilities	$ (22,536)	$ (6,276)
Gross Amounts Offset	15,859	4,082
Net Liabilities	(6,677)	(2,194)
Cash Collateral Offset	218	65
Net Amount Presented	(6,459)	(2,129)
Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(8,386)	(5,034)
Gross Amounts Offset	6,089	3,332
Net Liabilities	(2,297)	(1,702)
Cash Collateral Offset	218	65
Net Amount Presented	(2,079)	(1,637)
Non-trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(8,369)	(4,517)
Gross Amounts Offset	6,088	3,059
Net Liabilities	(2,281)	(1,458)
Cash Collateral Offset	218	65
Net Amount Presented	(2,063)	(1,393)
Trading Commodity Contract, Current
Offsetting Liabilities [Line Items]
Gross Liabilities	(17)	(517)
Gross Amounts Offset	1	273
Net Liabilities	(16)	(244)
Cash Collateral Offset	0	0
Net Amount Presented	(16)	(244)
Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(14,150)	(1,242)
Gross Amounts Offset	9,770	750
Net Liabilities	(4,380)	(492)
Cash Collateral Offset	0	0
Net Amount Presented	(4,380)	(492)
Non-trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(13,725)	(676)
Gross Amounts Offset	9,723	732
Net Liabilities	(4,002)	56
Cash Collateral Offset	0	0
Net Amount Presented	(4,002)	56
Trading Commodity Contract, Noncurrent
Offsetting Liabilities [Line Items]
Gross Liabilities	(425)	(566)
Gross Amounts Offset	47	18
Net Liabilities	(378)	(548)
Cash Collateral Offset	0	0
Net Amount Presented	$ (378)	$ (548)