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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Narrative (Details) | 3 Months Ended | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2021 USD ($) renewalOption | Mar. 31, 2020 USD ($) | Mar. 31, 2021 USD ($) renewalOption geographical_area | Mar. 31, 2020 USD ($) | Apr. 16, 2021 USD ($) | Oct. 01, 2020 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Nov. 13, 2018 USD ($)
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash, cash equivalents, and short-term investments | $ 561,700,000 |  | $ 561,700,000 |  |  |  |  |  |
Number of renewal options | renewalOption | 1 |  | 1 |  |  |  |  |  |
Operating lease right-of-use asset | $ 56,474,000 |  | $ 56,474,000 |  |  |  | $ 44,583,000 |  |
Restricted cash | 51,400,000 | $ 44,100,000 | 51,400,000 | $ 44,100,000 |  |  | 48,900,000 | $ 35,000,000.0 |
Restricted cash from initial capitalization of captive insurance company | 3,100,000 |  | 3,100,000 |  |  |  | 2,000,000.0 |  |
Additional cash restricted at the election of management for potential insurance claims | 45,100,000 |  | 45,100,000 |  |  |  | 43,100,000 |  |
Retained earnings | 2,762,735,000 |  | 2,762,735,000 |  |  |  | 3,010,012,000 |  |
Underwriting expenses | 2,300,000 | 6,000,000.0 | 2,800,000 | 14,700,000 |  |  |  |  |
Stop-loss program, maximum amount of claim |  |  | $ 50,000 |  |  |  |  |  |
Cumulative inflation rate before a country is considered highly inflationary (as a percent) |  |  | 100.00% |  |  |  |  |  |
Aggregate foreign currency gain (loss) | (2,400,000) | $ (3,400,000) | $ (4,200,000) | $ (2,800,000) |  |  |  |  |
Houston Assembly Facility |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Operating lease right-of-use asset | 15,500,000 |  | 15,500,000 |  |  |  |  |  |
Operating lease liability | $ 15,500,000 |  | $ 15,500,000 |  |  |  |  |  |
Operating Revenue | Geographic Concentration Risk | International Locations |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Concentration percentage | 5.20% | 8.20% | 4.90% | 8.00% |  |  |  |  |
Argentina |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Cash | $ 22,200,000 |  | $ 22,200,000 |  |  |  |  |  |
South America | Operating Revenue | Geographic Concentration Risk | International Locations | International Solutions |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Concentration percentage | 51.40% | 66.40% | 37.60% | 76.20% |  |  |  |  |
Intercompany Eliminations |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Premium revenues and expenses | $ 8,700,000 | $ 10,500,000 | $ 15,800,000 | $ 18,200,000 |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Lease contract term | 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) | geographical_area |  |  | 1 |  |  |  |  |  |
Minimum | Houston Assembly Facility |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Lease renewal term | 5 years |  | 5 years |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Lease contract term | 15 years |  | 15 years |  |  |  |  |  |
Insurance coverage deductibles range for claims which occur outside or inside the United States |  |  | $ 10,000,000 |  |  |  |  |  |
Maximum | Houston Assembly Facility |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Lease renewal term | 10 years |  | 10 years |  |  |  |  |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Accounts receivable, allowance for credit loss |  |  |  |  |  | $ 1,600,000 |  |  |
Retained earnings |  |  |  |  |  | $ (1,300,000) |  |  |
Acquisition of Drilling Technology Companies |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Restricted cash | $ 3,200,000 |  | $ 3,200,000 |  |  |  | $ 3,600,000 |  |
2018 Credit Facility |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  | $ 750,000,000.0
Borrowings outstanding | 0 |  | 0 |  |  |  |  |  |
Available borrowing capacity | $ 750,000,000.0 |  | $ 750,000,000.0 |  |  |  |  |  |
2018 Credit Facility | Letters of Credit |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  |  |  |  |  | $ 75,000,000.0
2018 Credit Facility, Due November 2025 | Subsequent Event |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  | $ 680,000,000.0 |  |  |  |
2018 Credit Facility, Due November 2024 | Subsequent Event |  |  |  |  |  |  |  |  |
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |
Maximum borrowing capacity |  |  |  |  | $ 70,000,000.0 |  |  |  |

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