| Schedule of Quarterly Financial Information [Table Text Block] |
The following table summarizes quarterly financial data for the years ended December 31, 2015 and 2014: | | | | | | | | | | | | | | | | | | | | | | First Quarter | | Second Quarter | | Third Quarter | | Fourth Quarter | | Total | | (Thousands of Dollars, Except Per Unit Data) | 2015: | | | | | | | | | | Revenues | $ | 554,944 |
| | $ | 570,611 |
| | $ | 493,566 |
| | $ | 464,919 |
| | $ | 2,084,040 |
| Operating income | $ | 99,281 |
| | $ | 92,405 |
| | $ | 100,994 |
| | $ | 98,024 |
| | $ | 390,704 |
| Income from continuing operations | $ | 127,125 |
| | $ | 54,325 |
| | $ | 65,016 |
| | $ | 59,480 |
| | $ | 305,946 |
| Income from discontinued operations, net of tax | 774 |
| | — |
| | — |
| | — |
| | 774 |
| Net income | $ | 127,899 |
| | $ | 54,325 |
| | $ | 65,016 |
| | $ | 59,480 |
| | $ | 306,720 |
| Net income per unit applicable to limited partners: | | | | | | | | | | Continuing operations | $ | 1.46 |
| | $ | 0.54 |
| | $ | 0.68 |
| | $ | 0.61 |
| | $ | 3.29 |
| Discontinued operations | 0.01 |
| | — |
| | — |
| | — |
| | 0.01 |
| Total | $ | 1.47 |
| | $ | 0.54 |
| | $ | 0.68 |
| | $ | 0.61 |
| | $ | 3.30 |
| Cash distributions per unit applicable to limited partners | $ | 1.095 |
| | $ | 1.095 |
| | $ | 1.095 |
| | $ | 1.095 |
| | $ | 4.380 |
| | | | | | | | | | | 2014: | | | | | | | | | | Revenues | $ | 849,213 |
| | $ | 749,745 |
| | $ | 794,422 |
| | $ | 681,738 |
| | $ | 3,075,118 |
| Operating income | $ | 81,103 |
| | $ | 89,354 |
| | $ | 95,098 |
| | $ | 81,346 |
| | $ | 346,901 |
| Income from continuing operations | $ | 42,996 |
| | $ | 57,187 |
| | $ | 59,117 |
| | $ | 54,869 |
| | $ | 214,169 |
| (Loss) income from discontinued operations, net of tax | (3,359 | ) | | (1,788 | ) | | 2,831 |
| | (1,475 | ) | | (3,791 | ) | Net income | $ | 39,637 |
| | $ | 55,399 |
| | $ | 61,948 |
| | $ | 53,394 |
| | $ | 210,378 |
| Net income (loss) per unit applicable to limited partners: | | | | | | | | | | Continuing operations | $ | 0.40 |
| | $ | 0.58 |
| | $ | 0.61 |
| | $ | 0.55 |
| | $ | 2.14 |
| Discontinued operations | (0.04 | ) | | (0.02 | ) | | 0.03 |
| | (0.01 | ) | | (0.04 | ) | Total | $ | 0.36 |
| | $ | 0.56 |
| | $ | 0.64 |
| | $ | 0.54 |
| | $ | 2.10 |
| Cash distributions per unit applicable to limited partners | $ | 1.095 |
| | $ | 1.095 |
| | $ | 1.095 |
| | $ | 1.095 |
| | $ | 4.380 |
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