| In millions, except per share items | Dec-31-22 | Dec-31-21 | Dec-31-20 | Dec-31-19 | Dec-31-18 | Dec-31-17 | Dec-31-16 | Dec-31-15 |
| | 8-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K |
| Revenues | | | | | | | | |
| Total revenues | 159.5 | 78.5 | 83.3 | 82.4 | 14.2 | 12.9 | 103.8 | 50.9 |
| Total revenues growth | 103.4% | -5.9% | 1.2% | 479.8% | 10.0% | -87.5% | 103.7% | 296.6% |
| Operating expenses | | | | | | | | |
| General and administrative | 4.5 | | | | | | 4.4 | 3.1 |
| Depreciation and amortization | | | | | | | 5.6 | 5.8 |
| Total operating expenses | 67.5 | 31.2 | 34.1 | 33.2 | 37.8 | 42.8 | 44.3 | 38.5 |
| Operating income | 46.0 | 32.5 | 40.8 | 41.0 | 55.8 | 59.1 | 59.5 | 12.4 |
| Operating margin | 28.8% | 41.4% | 48.9% | 49.7% | 393.0% | 457.0% | 57.3% | 24.3% |
| Investment Income, Interest | | | | | | | 0.1 | 0.1 |
| Interest expense | | | | | | | -2.5 | -0.4 |
| Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest | 40.1 | 39.9 | 40.7 | 41.0 | 55.8 | 59.0 | 57.0 | 12.1 |
| Income tax expense | -0.1 | -0.2 | -0.2 | -0.2 | -0.1 | -0.1 | -0.2 | -4.0 |
| Tax rate | 0.2% | 0.5% | 0.5% | 0.5% | 0.2% | 0.2% | 0.4% | 33.1% |
| Income from equity method investee | 0.6 | | | | | | | |
| Net income | 40.7 | 40.4 | 41.1 | 41.5 | 55.7 | 58.9 | 56.8 | 16.1 |
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| Net income attributable to partners' ownership interests | | | | | | | | |
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| Earnings per limited partner unit (basic and diluted) | | | | | | | | |
| Common units, basic (in dollars per share) | | | | | | | | |
| Common units, diluted (in dollars per share) | | | | | | | | |
| Weighted average limited partner units outstanding (basic and diluted) | | | | | | | | |
| Common units, basic (in shares) | | | | | | | | |
| Common units, diluted (in shares) | | | | | | | | |
| Share-based Payment Arrangement, Expense | | | | | | | | |