Condensed Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
Mar. 31, 2012	Mar. 31, 2011
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 80,539	$ 101,237
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	26,173	26,554
Deferred income taxes, net	12,229	(1,627)
Provision for uncollectible accounts	5,819	8,626
Other, net	(10,663)	3,524
Net change in:
Accounts receivable and accrued utility revenues	(62,385)	(137,223)
Inventories	80,623	102,184
Deferred fuel and power costs	14,039	43,281
Accounts payable	(28,996)	5,510
Other current assets	786	3,277
Other current liabilities	11,202	9,665
Net cash provided by operating activities	129,366	165,008
CASH FLOWS FROM INVESTING ACTIVITIES:
Expenditures for property, plant and equipment	(49,784)	(37,658)
Net costs of property, plant and equipment disposals	(1,541)	(1,220)
Decrease in restricted cash	48	702
Net cash used by investing activities	(51,277)	(38,176)
CASH FLOWS FROM FINANCING ACTIVITIES:
Payment of dividends	(39,004)	(39,348)
Decrease in bank loans	0	(17,000)
Other	241	670
Net cash used by financing activities	(38,763)	(55,678)
Cash and cash equivalents increase	39,326	71,154
CASH AND CASH EQUIVALENTS:
End of period	46,593	75,472
Beginning of period	7,267	4,318
Increase	$ 39,326	$ 71,154

Name:	us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents. Includes cash and cash equivalents associated with the entity's continuing operations. Excludes cash and cash equivalents associated with the disposal group (and discontinued operation).