Details of Selected Consolidated Balance Sheet Captions (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011
Schedule of Accounts Receivable [Abstract]
Billed accounts receivable	$ 177,953	$ 216,145
Unbilled revenue	42,694	48,006
Other accounts receivable	23,304	16,592
Total accounts receivable	243,951	280,743
Less: allowance for doubtful accounts	9,425	7,440
Net accounts receivable	234,526	273,303
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	24,707	18,344
Deferred gas cost	31,359	33,976
Taxes receivable	1,291	9,215
Current deferred tax asset	27,091	76,725
Prepaid expenses	17,114	22,499
Current portion of leased assets receivable	168	2,013
Materials and supplies	5,872	4,113
Asset held for sale	154,571	140,571
Other	10,609	9,015
Total other current assets	272,782	316,471
Schedule of Other Current Liabilities [Abstract]
Customer deposits	100,926	106,743
Accrued employee costs	37,675	38,558
Deferred gas costs	23,072	8,130
Accrued interest	34,451	37,557
Liabilitied from risk management activities current	85,381	15,453
Taxes payable	64,319	57,853
Pension and postretirement obligations	39,625	33,036
Regulatory cost of removal accrual	78,525	35,078
Liabilities held for sale	11,573	18,445
Other	14,118	16,710
Total	489,665	367,563
Schedule of Other Noncurrent Assets [Abstract]
Marketable securities	64,398	52,633
Regulatory assets	334,551	278,920
Deferred financing costs	35,101	35,149
Assets from risk management activities noncurrent	2,283	998
Other	14,929	16,093
Total	451,262	383,793
Schedule of Other Noncurrent Liabilities [Abstract]
Postretirement obligations	221,231	202,709
Retirement plan obligations	235,965	236,227
Customer advances for construction	12,937	13,967
Regulatory liabilities	5,638	13,823
Asset retirement obligation	10,394	13,574
Liabilities from risk management activities noncurrent	9,206	78,089
Other	12,555	6,306
Total	507,926	564,695
Schedule of Property, Plant and Equipment [Abstract]
Production plant	5,020	7,412
Storage plant	232,260	198,422
Transmission plant	1,185,007	1,126,509
Distribution plant	4,680,877	4,496,263
General plant	717,568	737,850
Intangible plant	39,626	41,096
Total property, plant and equipment	6,860,358	6,607,552
Construction in progress	274,112	209,242
Total property, plant and equipment and construction in progress	7,134,470	6,816,794
Less accumulated depreciation and amortization	1,658,866	1,668,876
Net property, plant and equipment	$ 5,475,604	$ 5,147,918