| In millions, except per share items | Jun-30-17 | Mar-31-17 | Dec-31-16 | Sep-30-16 | Jun-30-16 | Mar-31-16 | Dec-31-15 | Sep-30-15 |
| | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K | 10-Q |
| Revenues: | | | | | | | | |
| Revenues: Affiliate | | | | | | | | |
| Revenues: Third-party | | | | | | | | |
| Total revenues | 627.5 | 604.7 | 606.8 | 569.3 | 578.6 | 468.0 | 575.9 | 680.7 |
| Total revenues growth | 8.5% | 29.2% | 5.4% | -16.4% | -21.4% | -22.9% | -23.3% | -25.7% |
| Operating costs and expenses: | | | | | | | | |
| Cost of products sold: | | | | | | | | |
| Affiliate | 137.2 | 122.7 | | 128.8 | 123.9 | 95.1 | | 156.4 |
| Third-party | 403.2 | 394.6 | 399.6 | 366.7 | 380.4 | 300.4 | 376.7 | 445.2 |
| Operating and maintenance expenses | 47.3 | 44.8 | 43.8 | 43.5 | 43.0 | 44.7 | 44.6 | 45.9 |
| Selling, general and administrative expenses | 8.0 | 6.7 | 5.5 | 6.5 | 6.0 | 5.4 | 6.5 | 5.8 |
| Gain on disposal of assets, net | -2.9 | -0.3 | -0.1 | -0.1 | -0.8 | -0.1 | 0.0 | 0.0 |
| Depreciation and amortization | 9.8 | 9.7 | 9.8 | 10.6 | 9.6 | 9.3 | 9.6 | 9.0 |
| Total operating costs and expenses | 602.5 | 578.3 | 579.8 | 556.0 | 562.0 | 454.9 | 561.6 | 662.2 |
| Operating income | 25.0 | 26.4 | 27.0 | 13.3 | 16.6 | 13.2 | 14.3 | 18.4 |
| Operating margin | 4.0% | 4.4% | 4.5% | 2.3% | 2.9% | 2.8% | 2.5% | 2.7% |
| |
| Other income (expense): | | | | | | | | |
| Interest and debt expense | 6.6 | 6.6 | 6.4 | 6.1 | 6.4 | 7.1 | 6.7 | 6.2 |
| Other income (expense), net | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | -0.1 | 0.0 | 0.0 |
| Net income before income taxes | 18.5 | 19.8 | 20.7 | 7.1 | 10.2 | 6.0 | 7.7 | 12.2 |
| Benefit (provision) for income taxes | 0.3 | 0.1 | -0.1 | 0.3 | 0.2 | 0.3 | -0.3 | 0.0 |
| Net income | 18.7 | 19.9 | -65.9 | 6.9 | 10.0 | 5.8 | -57.4 | 12.2 |
| Less net loss attributable to General Partner | | | | -7.2 | -7.9 | -8.3 | -6.9 | -4.3 |
| Net income attributable to limited partners | 18.7 | 19.9 | 20.8 | 14.0 | 17.9 | 14.0 | 14.8 | 16.5 |
| |
| Earnings Per Share [Abstract] | | | | | | | | |
| Earnings Per Share, Basic | | | | | | | | |
| Earnings Per Share, Diluted | | | | | | | | |
| Earnings Per Share, Basic and Diluted | | | | | | | | |
| Weighted Average Number of Shares Outstanding, Basic | | | | | | | | |
| Weighted Average Number of Shares Outstanding, Diluted | | | | | | | | |
| Weighted Average Number of Shares Outstanding, Basic and Diluted | | | | | | | | |
| Distribution Made to Limited Partner, Distributions Declared, Per Unit | 0.0 | 0.0 | | 0.0 | 0.0 | 0.0 | | 0.0 |