| Fair Value, Measurement Inputs, Disclosure [Text Block] |
The following assets and liabilities are measured at fair value on a recurring basis: | | | | | | | | | | | | | | | | | | December 31, 2017 | | Level 1 | | Level 2 | | Level 3 | | Total | | (Thousands of Dollars) | Assets: | | | | | | | | Other current assets: | | | | | | | | Product imbalances | $ | 3,890 |
| | $ | — |
| | $ | — |
| | $ | 3,890 |
| Liabilities: | | | | | | | | Accrued liabilities: | | | | | | | | Product imbalances | $ | (1,534 | ) | | $ | — |
| | $ | — |
| | $ | (1,534 | ) | Commodity derivatives | (878 | ) | | — |
| | — |
| | (878 | ) | Interest rate swaps | — |
| | (5,394 | ) | | — |
| | (5,394 | ) | Other long-term liabilities: | | | | | | | | Interest rate swaps | — |
| | (4,594 | ) | | — |
| | (4,594 | ) | Total | $ | (2,412 | ) | | $ | (9,988 | ) | | $ | — |
| | $ | (12,400 | ) |
| | | | | | | | | | | | | | | | | | December 31, 2016 | | Level 1 | | Level 2 | | Level 3 | | Total | | (Thousands of Dollars) | Assets: | | | | | | | | Other current assets: | | | | | | | | Product imbalances | $ | 1,551 |
| | $ | — |
| | $ | — |
| | $ | 1,551 |
| Commodity derivatives | — |
| | 155 |
| | — |
| | 155 |
| Other long-term assets, net: | | | | | | | | Interest rate swaps | — |
| | 1,314 |
| | — |
| | 1,314 |
| Total | $ | 1,551 |
| | $ | 1,469 |
| | $ | — |
| | $ | 3,020 |
| Liabilities: | | | | | | | | Accrued liabilities: | | | | | | | | Product imbalances | $ | (1,577 | ) | | $ | — |
| | $ | — |
| | $ | (1,577 | ) | Commodity derivatives | (4,887 | ) | | (165 | ) | | — |
| | (5,052 | ) | Other long-term liabilities: | | | | | | | | Guarantee liability | — |
| | — |
| | (1,230 | ) | | (1,230 | ) | Interest rate swaps | — |
| | (2,632 | ) | | — |
| | (2,632 | ) | Total | $ | (6,464 | ) | | $ | (2,797 | ) | | $ | (1,230 | ) | | $ | (10,491 | ) |
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| Fair Value and Carrying Value of Debt and Note Receivable [Text Block] |
The estimated fair values and carrying amounts of the long-term debt, including the current portion, and the Axeon Term Loan were as follows: | | | | | | | | | | | | | | December 31, 2017 | | December 31, 2016 | | Long-term Debt | | Long-term Debt | | Axeon Term Loan | | (Thousands of Dollars) | Fair value | $ | 3,677,622 |
| | $ | 3,084,762 |
| | $ | 110,000 |
| Carrying amount | $ | 3,613,059 |
| | $ | 3,014,364 |
| | $ | 110,000 |
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