| Schedule of Segment Reporting Information, by Segment [Table Text Block] |
Results of operations for the reportable segments were as follows: | | | | | | | | | | | | | | | | | | Three Months Ended September 30, | | Nine Months Ended September 30, | | 2018 | | 2017 | | 2018 | | 2017 | | (Thousands of Dollars) | Revenues: | | | | | | | | Pipeline | $ | 162,843 |
| | $ | 137,426 |
| | $ | 449,909 |
| | $ | 385,406 |
| Storage: | | | | | | | | Third parties | 152,411 |
| | 155,677 |
| | 461,109 |
| | 453,995 |
| Intersegment | 1,719 |
| | 2,394 |
| | 5,799 |
| | 10,066 |
| Total storage | 154,130 |
| | 158,071 |
| | 466,908 |
| | 464,061 |
| Fuels marketing | 175,109 |
| | 147,463 |
| | 541,430 |
| | 524,083 |
| Consolidation and intersegment eliminations | (1,719 | ) | | (2,394 | ) | | (5,799 | ) | | (10,066 | ) | Total revenues | $ | 490,363 |
| | $ | 440,566 |
| | $ | 1,452,448 |
| | $ | 1,363,484 |
| | | | | | | | | Operating income: | | | | | | | | Pipeline | $ | 77,021 |
| | $ | 61,119 |
| | $ | 197,794 |
| | $ | 179,015 |
| Storage | 46,459 |
| | 59,323 |
| | 146,904 |
| | 169,131 |
| Fuels marketing | 1,890 |
| | (1,532 | ) | | 11,114 |
| | 3,897 |
| Consolidation and intersegment eliminations | — |
| | (1 | ) | | — |
| | — |
| Total segment operating income | 125,370 |
| | 118,909 |
| | 355,812 |
| | 352,043 |
| General and administrative expenses | 27,817 |
| | 25,003 |
| | 75,572 |
| | 83,202 |
| Other depreciation and amortization expense | 2,276 |
| | 2,189 |
| | 6,645 |
| | 6,581 |
| Total operating income | $ | 95,277 |
| | $ | 91,717 |
| | $ | 273,595 |
| | $ | 262,260 |
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| Reconciliation of Assets from Segment to Consolidated [Table Text Block] |
Total assets by reportable segment were as follows: | | | | | | | | | | September 30, 2018 | | December 31, 2017 | | (Thousands of Dollars) | Pipeline | $ | 3,606,413 |
| | $ | 3,492,417 |
| Storage | 2,787,886 |
| | 2,735,563 |
| Fuels marketing | 132,497 |
| | 118,746 |
| Total segment assets | 6,526,796 |
| | 6,346,726 |
| Other partnership assets | 194,162 |
| | 188,507 |
| Total consolidated assets | $ | 6,720,958 |
| | $ | 6,535,233 |
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