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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Oct. 01, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | $ 1,569,162 | $ 1,467,374 | $ 1,418,978 | $ 1,236,643 |
Balance as of end of period |  | 1,572,197 | 1,449,962 | 1,572,197 | 1,449,962 |
Investments in Equity Securities |  |  |  |  |  |
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | 0 | 0 | 3,446 | 11,044 |
Other comprehensive (loss), before reclassifications, net of tax |  |  | (2,364) |  | (25,055) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 0 | 0 | 0 | 11,647 |
Net current-period other comprehensive income, net of tax |  |  | (2,364) |  | (13,408) |
Reclassification to retained earnings | [1] |  |  |  |  | $ (3,446)
Balance as of end of period |  | 0 | (2,364) | 0 | (2,364) |
Tax on other comprehensive income (loss) before reclassifications |  |  | 854 |  | 9,071 |
Tax on amounts reclassified from accumulated other comprehensive income |  | 0 | 0 | 0 | (858) |
Tax on net current-period other comprehensive income (loss) |  |  | 854 |  | 8,213 |
Postemployment Benefit Obligation |  |  |  |  |  |
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | (15,517) | (13,788) | (16,056) | (14,300) |
Other comprehensive (loss), before reclassifications, net of tax |  |  | 0 |  | 0 |
Amounts reclassified from accumulated other comprehensive income, net of tax | [2] | 305 | 272 | 844 | 784 |
Net current-period other comprehensive income, net of tax |  |  | 272 |  | 784 |
Reclassification to retained earnings |  |  |  |  |  | 0
Balance as of end of period |  | (15,212) | (13,516) | (15,212) | (13,516) |
Tax on other comprehensive income (loss) before reclassifications |  |  | 0 |  | 0 |
Tax on amounts reclassified from accumulated other comprehensive income |  | (118) | (104) | (333) | (344) |
Tax on net current-period other comprehensive income (loss) |  |  | (104) |  | (344) |
Total |  |  |  |  |  |
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | (15,517) | (13,788) | (12,610) | (3,256) |
Other comprehensive (loss), before reclassifications, net of tax |  |  | (2,364) |  | (25,055) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 305 | 272 | 844 | 12,431 |
Net current-period other comprehensive income, net of tax |  |  | (2,092) |  | (12,624) |
Reclassification to retained earnings |  |  |  |  |  | $ (3,446)
Balance as of end of period |  | (15,212) | (15,880) | (15,212) | (15,880) |
Tax on other comprehensive income (loss) before reclassifications |  |  | 854 |  | 9,071 |
Tax on amounts reclassified from accumulated other comprehensive income |  | $ (118) | (104) | $ (333) | (1,202) |
Tax on net current-period other comprehensive income (loss) |  |  | $ 750 |  | $ 7,869 |
[1] | Due to the adoption of ASU No. 2016-01, an amendment to ASC 825, Financial Instruments. See Note 2. Summary of Significant Accounting Policies - Recently Adopted Updates to the Accounting Standards Codification section for more details.
[2] | Included in the computation of net periodic pension cost, a component of operations and maintenance expense on the Unaudited Condensed Consolidated Statements of Operations.

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