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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2018 | Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |
Beginning Balance |  | $ 1,236,643 | $ 1,166,591
Reclassifications of certain income tax effects to retained earnings | [1] | 3,300 |
Ending Balance |  | 1,418,978 | 1,236,643
Unrealized gain (loss) on available for sale securities |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |
Beginning Balance |  | 11,044 | 4,198
Other comprehensive (loss), before reclassifications, net of tax |  | (19,245) | 10,019
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 11,647 | (3,173)
Net current-period other comprehensive (loss), net of tax |  | (7,598) | 6,846
Reclassifications of certain income tax effects to retained earnings | [1] | 0 |
Ending Balance |  | 3,446 | 11,044
Tax on other comprehensive income before reclassifications |  | (6,973) | 6,593
Tax on amounts reclassified from accumulated other comprehensive income |  | 858 | (2,192)
Tax on net current-period other comprehensive income (loss) |  | (6,115) | 4,401
Adjustment to postemployment benefit obligation |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |
Beginning Balance |  | (14,300) | (19,353)
Other comprehensive (loss), before reclassifications, net of tax |  | 464 | 3,783
Amounts reclassified from accumulated other comprehensive income, net of tax |  | 1,056 | 1,270
Net current-period other comprehensive (loss), net of tax |  | 1,520 | 5,053
Reclassifications of certain income tax effects to retained earnings | [1] | 3,276 |
Ending Balance |  | (16,056) | (14,300)
Tax on other comprehensive income before reclassifications |  | 125 | 2,619
Tax on amounts reclassified from accumulated other comprehensive income |  | 448 | 868
Tax on net current-period other comprehensive income (loss) |  | 573 | 3,487
Total |  |  |
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] |  |  |
Beginning Balance |  | (3,256) | (15,155)
Other comprehensive (loss), before reclassifications, net of tax |  | (18,781) | 13,802
Amounts reclassified from accumulated other comprehensive income, net of tax | [2] | 12,703 | (1,903)
Net current-period other comprehensive (loss), net of tax |  | (6,078) | 11,899
Reclassifications of certain income tax effects to retained earnings | [1] | 3,276 |
Ending Balance |  | (12,610) | (3,256)
Tax on other comprehensive income before reclassifications |  | (6,848) | 9,212
Tax on amounts reclassified from accumulated other comprehensive income |  | 1,306 | (1,324)
Tax on net current-period other comprehensive income (loss) |  | $ (5,542) | $ 7,888
[1] | Due to the adoption of ASU No. 2018-02, an amendment to ASC 740, Income Taxes. 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 O&M expense on the Consolidated Statements of Operations. For more details, see Note 10. Employee Benefit Plans.

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