| **Derivative Accounting - Derivative Instruments in the Balance Sheet (Details) - USD ($)** | Dec. 31, 2017 | Dec. 31, 2016 |
|---------------------------------------------------------------------------------------------|---------------|---------------|
| Commodity Contracts                                                                         |          |          |
| **Assets**                                                                                  |          |          |
| Gross Recognized Derivatives                                                                | $ 6,719,000   | $ 54,798,000  |
| Amounts Offset                                                                              | (5,037,000)   | (35,103,000)  |
| Net Recognized Derivatives                                                                  | 1,682,000     | 19,695,000    |
| Other                                                                                       | 300,000       | 0             |
| Amount Reported on Balance Sheet                                                            | 1,982,000     | 19,695,000    |
| **Liabilities**                                                                             |          |          |
| Gross Recognized Derivatives                                                                | (97,938,000)  | (104,123,000) |
| Amounts Offset                                                                              | 5,037,000     | 35,103,000    |
| Net Recognized Derivatives                                                                  | (92,901,000)  | (69,020,000)  |
| Other                                                                                       | (3,521,000)   | (4,054,000)   |
| Amount Reported on Balance Sheet                                                            | (96,422,000)  | (73,074,000)  |
| **Assets and Liabilities**                                                                  |          |          |
| Gross Recognized Derivatives                                                                | (91,219,000)  | (49,325,000)  |
| Amounts Offset                                                                              | 0             | 0             |
| Net Recognized Derivatives                                                                  | (91,219,000)  | (49,325,000)  |
| Other                                                                                       | (3,221,000)   | (4,054,000)   |
| Amount Reported on Balance Sheet                                                            | (94,440,000)  | (53,379,000)  |
| Commodity Contracts | Current Assets                                                        |          |          |
| **Assets**                                                                                  |          |          |
| Gross Recognized Derivatives                                                                | 5,427,000     | 48,094,000    |
| Amounts Offset                                                                              | (3,796,000)   | (28,400,000)  |
| Net Recognized Derivatives                                                                  | 1,631,000     | 19,694,000    |
| Other                                                                                       | 300,000       | 0             |
| Amount Reported on Balance Sheet                                                            | 1,931,000     | 19,694,000    |
| Commodity Contracts | Investments and Other Assets                                          |          |          |
| **Assets**                                                                                  |          |          |
| Gross Recognized Derivatives                                                                | 1,292,000     | 6,704,000     |
| Amounts Offset                                                                              | (1,241,000)   | (6,703,000)   |
| Net Recognized Derivatives                                                                  | 51,000        | 1,000         |
| Other                                                                                       | 0             | 0             |
| Amount Reported on Balance Sheet                                                            | 51,000        | 1,000         |
| Commodity Contracts | Current Liabilities                                                   |          |          |
| **Liabilities**                                                                             |          |          |
| Gross Recognized Derivatives                                                                | (59,527,000)  | (50,182,000)  |
| Amounts Offset                                                                              | 3,796,000     | 28,400,000    |
| Net Recognized Derivatives                                                                  | (55,731,000)  | (21,782,000)  |
| Other                                                                                       | (3,521,000)   | (4,054,000)   |
| Amount Reported on Balance Sheet                                                            | (59,252,000)  | (25,836,000)  |
| Commodity Contracts | Deferred Credits and Other                                            |          |          |
| **Liabilities**                                                                             |          |          |
| Gross Recognized Derivatives                                                                | (38,411,000)  | (53,941,000)  |
| Amounts Offset                                                                              | 1,241,000     | 6,703,000     |
| Net Recognized Derivatives                                                                  | (37,170,000)  | (47,238,000)  |
| Other                                                                                       | 0             | 0             |
| Amount Reported on Balance Sheet                                                            | (37,170,000)  | (47,238,000)  |
| Designated as Hedging Instruments                                                           |          |          |
| **Liabilities**                                                                             |          |          |
| Amount Reported on Balance Sheet                                                            | $ 0           | $ (2,000,000) |

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Represents the fair value of derivative asset, net of liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.
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Derivative Asset (Liability), Fair Value, Gross Liability (Asset) and Obligation to Return and Right to Reclaim Cash, Offset
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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.
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Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement.
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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.
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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.
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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.
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| - Definition
Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.
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| - Definition
Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.
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| - Definition
Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.
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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.
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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and right to receive cash collateral under master netting arrangements.
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Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement.
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