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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Concentration of Risk (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Major customers |  |  |  |  |  |  |  |  |  |  |
Operating revenue | $ 696,825 | $ 648,872 | $ 577,484 | $ 564,087 | $ 532,304 | $ 498,564 | $ 405,283 | $ 368,590 | $ 2,487,268 | $ 1,804,741 | $ 1,624,232
Operating revenues | Customer concentration | EOG Resources, Inc. |  |  |  |  |  |  |  |  |  |  |
Major customers |  |  |  |  |  |  |  |  |  |  |
Operating revenue |  |  |  |  |  |  |  |  | $ 258,194 | $ 163,582 | $ 124,262
Receivables | Credit concentration | EOG Resources, Inc. |  |  |  |  |  |  |  |  |  |  |
Major customers |  |  |  |  |  |  |  |  |  |  |
Concentration percentage |  |  |  |  |  |  |  |  | 8.80% |  |
Receivables | Credit concentration | Occidental Oil and Gas Corporation |  |  |  |  |  |  |  |  |  |  |
Major customers |  |  |  |  |  |  |  |  |  |  |
Concentration percentage |  |  |  |  |  |  |  |  | 4.70% |  |

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