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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Narrative (Details) | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2020 USD ($) | Mar. 31, 2019 USD ($) | Mar. 31, 2020 USD ($) greographical_area | Mar. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($)
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash, cash equivalents, and short-term investments | $ 381,700,000 |  | $ 381,700,000 |  |  |
Cash and cash equivalents | 336,089,000 | $ 243,912,000 | 336,089,000 | $ 243,912,000 | $ 347,943,000 | $ 284,355,000
Restricted cash | 44,100,000 | 34,200,000 | 44,100,000 | 34,200,000 | 35,000,000.0 | $ 41,800,000
Restricted cash from initial capitalization of captive insurance company | 2,000,000.0 |  | 2,000,000.0 |  | 2,000,000 |
Additional cash restricted at the election of management for potential insurance claims | 39,100,000 |  | 39,100,000 |  | 30,000,000.0 |
Restricted cash from other purposes | 1,100,000 |  | 1,100,000 |  |  |
Underwriting expenses | 6,000,000.0 |  | $ 14,700,000 |  |  |
Cumulative inflation rate before a country is considered highly inflationary (as a percent) |  |  | 100.00% |  |  |
Aggregate foreign currency gain (loss) | $ (3,400,000) | $ (700,000) | $ (2,800,000) | $ (4,600,000) |  |
Stop-loss program, maximum amount of claim |  |  | $ 50,000 |  |  |
Minimum |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Rental contract period | 1 year |  | 1 year |  |  |
Insurance coverage deductibles range for claims which occur outside or inside the United States |  |  | $ 1,000,000 |  |  |
Number of geographical areas operating in (more than) | greographical_area |  |  | 1 |  |  |
Maximum |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Rental contract period | 15 years |  | 15 years |  |  |
Insurance coverage deductibles range for claims which occur outside or inside the United States |  |  | $ 10,000,000 |  |  |
Intercompany Eliminations |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Premium revenues and expenses | $ 10,500,000 |  | 18,200,000 |  |  |
United States |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash and cash equivalents | 294,600,000 |  | 294,600,000 |  |  |
International Locations |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 41,500,000 |  | $ 41,500,000 |  |  |
International Locations | Operating Revenue | Geographic Concentration Risk |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Concentration percentage | 8.20% | 7.10% | 8.00% | 8.10% |  |
Argentina |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash | $ 33,700,000 |  | $ 33,700,000 |  |  |
International Land | South America | International Locations | Segment Operating Revenue | Geographic Concentration Risk |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Concentration percentage | 66.40% | 93.90% | 76.20% | 90.90% |  |
Acquisition of Drilling Technology Companies |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Restricted cash | $ 1,900,000 |  | $ 1,900,000 |  | $ 3,000,000.0 |

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