Condensed Consolidated Statements of Cash Flows (unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 118,428us-gaap_NetIncomeLoss	$ 121,396us-gaap_NetIncomeLoss
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	31,069	29,093
Deferred income taxes, net	(13,888)us-gaap_DeferredIncomeTaxExpenseBenefit	12,109us-gaap_DeferredIncomeTaxExpenseBenefit
Provision for uncollectible accounts	8,979us-gaap_ProvisionForDoubtfulAccounts	8,716us-gaap_ProvisionForDoubtfulAccounts
Other, net	2,521us-gaap_OtherNoncashIncomeExpense	414us-gaap_OtherNoncashIncomeExpense
Net change in:
Accounts receivable and accrued utility revenues	(154,515)	(173,176)
Inventories	75,686us-gaap_IncreaseDecreaseInInventories	69,032us-gaap_IncreaseDecreaseInInventories
Deferred fuel and power costs, net of changes in unsettled derivatives	55,760	(10,181)
Accounts payable	8,013	25,726
Other current assets	(921)	1,746
Other current liabilities	66,025	38,355
Net cash provided by operating activities	197,157	123,230
CASH FLOWS FROM INVESTING ACTIVITIES:
Expenditures for property, plant and equipment	(102,020)	(65,902)
Net costs of property, plant and equipment disposals	(3,694)	(2,948)
(Increase) decrease in restricted cash	(1,986)	3,181
Net cash used by investing activities	(107,700)	(65,669)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of dividends	(30,600)us-gaap_PaymentsOfDividendsCommonStock	(37,693)us-gaap_PaymentsOfDividendsCommonStock
Issuances of long-term debt	0	175,000
Repayments of long-term debt	0us-gaap_RepaymentsOfLongTermDebt	(175,000)us-gaap_RepaymentsOfLongTermDebt
Decrease in short-term borrowings	(55,800)	(11,000)
Other	582	663
Net cash used by financing activities	(85,818)	(48,030)
Cash and cash equivalents increase	3,639	9,531
CASH AND CASH EQUIVALENTS:
End of period	16,040	14,238
Beginning of period	$ 12,401	$ 4,707

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