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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Oct. 01, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | $ 1,497,050 | $ 1,347,770 | $ 1,418,978 | $ 1,236,643 |
Balance as of end of period |  | 1,569,162 | 1,467,374 | 1,569,162 | 1,467,374 |
Investments in Equity Securities |  |  |  |  |  |
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | 0 | 5,600 | 3,446 | 11,044 |
Other comprehensive (loss), before reclassifications, net of tax |  |  | (20,401) |  | (22,691) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 0 | 14,801 | 0 | 11,647 |
Net current-period other comprehensive income, net of tax |  |  | (5,600) |  | (11,044) |
Reclassification to retained earnings | [1] |  |  |  |  | $ (3,446)
Balance as of end of period |  | 0 | 0 | 0 | 0 |
Tax on other comprehensive income (loss) before reclassifications |  |  | (7,366) |  | (8,217) |
Tax on amounts reclassified from accumulated other comprehensive income |  | 0 | 3,036 | 0 | 858 |
Tax on net current-period other comprehensive income (loss) |  |  | (4,330) |  | (7,359) |
Postemployment Benefit Obligation |  |  |  |  |  |
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | (15,822) | (14,060) | (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 | 539 | 512 |
Net current-period other comprehensive income, net of tax |  |  | 272 |  | 512 |
Reclassification to retained earnings |  |  |  |  |  | 0
Balance as of end of period |  | (15,517) | (13,788) | (15,517) | (13,788) |
Tax on other comprehensive income (loss) before reclassifications |  |  | 0 |  | 0 |
Tax on amounts reclassified from accumulated other comprehensive income |  | (119) | 104 | (215) | 240 |
Tax on net current-period other comprehensive income (loss) |  |  | 104 |  | 240 |
Total |  |  |  |  |  |
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward] |  |  |  |  |  |
Balance as of beginning of period |  | (15,822) | (8,460) | (12,610) | (3,256) |
Other comprehensive (loss), before reclassifications, net of tax |  |  | (20,401) |  | (22,691) |
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 305 | 15,073 | 539 | 12,159 |
Net current-period other comprehensive income, net of tax |  |  | (5,328) |  | (10,532) |
Reclassification to retained earnings |  |  |  |  |  | $ (3,446)
Balance as of end of period |  | (15,517) | (13,788) | (15,517) | (13,788) |
Tax on other comprehensive income (loss) before reclassifications |  |  | (7,366) |  | (8,217) |
Tax on amounts reclassified from accumulated other comprehensive income |  | $ (119) | 3,140 | $ (215) | 1,098 |
Tax on net current-period other comprehensive income (loss) |  |  | $ (4,226) |  | $ (7,119) |
[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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