| Schedule of Segment Reporting Information, by Segment [Table Text Block] |
Results of operations for the reportable segments were as follows: | | | | | | | | | | | | | | | | | | Three Months Ended June 30, | | Six Months Ended June 30, | | 2018 | | 2017 | | 2018 | | 2017 | | (Thousands of Dollars) | Revenues: | | | | | | | | Pipeline | $ | 150,276 |
| | $ | 126,740 |
| | $ | 287,066 |
| | $ | 247,980 |
| Storage: | | | | | | | | Third parties | 155,445 |
| | 154,830 |
| | 308,698 |
| | 298,318 |
| Intersegment | 2,005 |
| | 3,729 |
| | 4,080 |
| | 7,672 |
| Total storage | 157,450 |
| | 158,559 |
| | 312,778 |
| | 305,990 |
| Fuels marketing | 180,483 |
| | 153,918 |
| | 366,321 |
| | 376,620 |
| Consolidation and intersegment eliminations | (2,005 | ) | | (3,729 | ) | | (4,080 | ) | | (7,672 | ) | Total revenues | $ | 486,204 |
| | $ | 435,488 |
| | $ | 962,085 |
| | $ | 922,918 |
| | | | | | | | | Operating income: | | | | | | | | Pipeline | $ | 62,979 |
| | $ | 52,868 |
| | $ | 120,773 |
| | $ | 117,896 |
| Storage | 44,184 |
| | 56,049 |
| | 100,445 |
| | 109,808 |
| Fuels marketing | 2,904 |
| | 289 |
| | 9,224 |
| | 5,429 |
| Consolidation and intersegment eliminations | 3 |
| | 1 |
| | — |
| | 1 |
| Total segment operating income | 110,070 |
| | 109,207 |
| | 230,442 |
| | 233,134 |
| General and administrative expenses | 27,981 |
| | 33,604 |
| | 47,755 |
| | 58,199 |
| Other depreciation and amortization expense | 2,251 |
| | 2,199 |
| | 4,369 |
| | 4,392 |
| Total operating income | $ | 79,838 |
| | $ | 73,404 |
| | $ | 178,318 |
| | $ | 170,543 |
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| Reconciliation of Assets from Segment to Consolidated [Table Text Block] |
Total assets by reportable segment were as follows: | | | | | | | | | | June 30,
2018 | | December 31,
2017 | | (Thousands of Dollars) | Pipeline | $ | 3,581,710 |
| | $ | 3,492,417 |
| Storage | 2,754,290 |
| | 2,735,563 |
| Fuels marketing | 118,119 |
| | 118,746 |
| Total segment assets | 6,454,119 |
| | 6,346,726 |
| Other partnership assets | 192,850 |
| | 188,507 |
| Total consolidated assets | $ | 6,646,969 |
| | $ | 6,535,233 |
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