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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  | $ 1,551,717 | $ 1,418,978 |
Reclassification of certain income tax effects to retained earnings |  | 1,844,692 | 1,418,978 | $ 1,236,643
Ending Balance |  | 1,844,692 | 1,551,717 |
Investments in Equity Securities |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  | 0 | 3,446 |
Other comprehensive (loss), before reclassifications, net of tax |  | 0 | 0 |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 0 | 0 |
Net current-period other comprehensive (loss), net of tax |  | 0 | 0 |
Reclassification of certain income tax effects to retained earnings |  | 0 | 0 |
Ending Balance |  | 0 | 0 |
Tax on other comprehensive income before reclassifications |  | 0 | 0 |
Tax on amounts reclassified from accumulated other comprehensive income |  | 0 | 0 |
Tax on net current-period other comprehensive income |  | 0 | 0 |
Investments in Equity Securities | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01 |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | (3,446) |
Reclassification of certain income tax effects to retained earnings |  |  | (3,446) |
Cash Flow Hedges |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  | 0 | 0 |
Other comprehensive (loss), before reclassifications, net of tax |  | (10,505) | 0 |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 108 | 0 |
Net current-period other comprehensive (loss), net of tax |  | (10,397) | 0 |
Reclassification of certain income tax effects to retained earnings |  | (10,397) | 0 |
Ending Balance |  | (10,397) | 0 |
Tax on other comprehensive income before reclassifications |  | 3,203 | 0 |
Tax on amounts reclassified from accumulated other comprehensive income |  | (32) | 0 |
Tax on net current-period other comprehensive income |  | 3,171 | 0 |
Cash Flow Hedges | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01 |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | 0 |
Reclassification of certain income tax effects to retained earnings |  |  | 0 |
Postemployment Benefit Obligation |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  | (31,787) | (16,056) |
Other comprehensive (loss), before reclassifications, net of tax |  | (4,882) | (16,978) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 2,751 | 1,247 |
Net current-period other comprehensive (loss), net of tax |  | (2,131) | (15,731) |
Reclassification of certain income tax effects to retained earnings |  | (33,918) | (31,787) |
Ending Balance |  | (33,918) | (31,787) |
Tax on other comprehensive income before reclassifications |  | 1,235 | 6,557 |
Tax on amounts reclassified from accumulated other comprehensive income |  | (668) | (451) |
Tax on net current-period other comprehensive income |  | 567 | 6,106 |
Postemployment Benefit Obligation | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01 |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | 0 |
Reclassification of certain income tax effects to retained earnings |  |  | 0 |
Total |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  | (31,787) | (12,610) |
Other comprehensive (loss), before reclassifications, net of tax |  | (15,387) | (16,978) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 2,859 | 1,247 |
Net current-period other comprehensive (loss), net of tax |  | (12,528) | (15,731) |
Reclassification of certain income tax effects to retained earnings |  | (31,787) | (31,787) | $ (3,256)
Ending Balance |  | (44,315) | (31,787) |
Tax on other comprehensive income before reclassifications |  | 4,438 | 6,557 |
Tax on amounts reclassified from accumulated other comprehensive income |  | (700) | (451) |
Tax on net current-period other comprehensive income |  | $ 3,738 | 6,106 |
Total | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-01 |  |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |  |
Beginning Balance | [1] |  | (3,446) |
Reclassification of certain income tax effects to retained earnings | [1] |  | $ (3,446) |
[1] | See Note 2. Summary of Significant Accounting Policies - Recently Adopted Updates to the Accounting Standards Codification section for more details.

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