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.