Details of Selected Consolidated Balance Sheet Captions (Details) (USD $) In Thousands	Sep. 30, 2011	Sep. 30, 2010
Schedule of Accounts Receivable [Abstract]
Billed accounts receivable	$ 216,145	$ 223,129
Unbilled revenue	48,006	47,423
Other accounts receivable	16,592	15,356
Total accounts receivable	280,743	285,908
Less: allowance for doubtful accounts	7,440	12,701
Net accounts receivable	273,303	273,207
Schedule of Other Current Assets [Abstract]
Assets from risk management activities current	18,344	20,575
Deferred gas cost	33,976	22,701
Taxes receivable	9,215	19,382
Current deferred tax asset	76,725	53,926
Prepaid expenses	22,499	24,754
Current portion of leased assets receivable	2,013	2,973
Materials and supplies	4,113	3,940
Asset held for sale	140,571	0
Other	9,015	2,744
Total other current assets	316,471	150,995
Schedule of Other Current Liabilities [Abstract]
Customer deposits	106,743	114,215
Accrued employee costs	38,558	40,642
Deferred gas costs	8,130	43,333
Accrued interest	37,557	42,901
Liabilitied from risk management activities current	15,453	49,673
Taxes payable	57,853	56,616
Pension and postretirement obligations	33,036	14,815
Regulatory cost of removal accrual	35,078	30,953
Liabilities held for sale	18,445
Other	16,710	20,492
Total	367,563	413,640
Schedule of Other Noncurrent Assets [Abstract]
Marketable securities	52,633	41,466
Regulatory assets	278,920	254,809
Deferred financing costs	35,149	35,761
Assets from risk management activities noncurrent	998	937
Other	16,093	22,403
Total	383,793	355,376
Schedule of Other Noncurrent Liabilities [Abstract]
Postretirement obligations	202,709	167,899
Retirement plan obligations	236,227	207,234
Customer advances for construction	13,967	15,466
Regulatory liabilities	13,823	6,112
Asset retirement obligation	13,574	11,432
Uncertain tax positions	0	6,731
Liabilities from risk management activities noncurrent	78,089	8,924
Other	6,306	6,366
Total	564,695	430,164
Schedule of Property, Plant and Equipment [Abstract]
Production plant	7,412	17,360
Storage plant	198,422	193,155
Transmission plant	1,126,509	1,108,398
Distribution plant	4,496,263	4,339,277
General plant	737,850	671,953
Intangible plant	41,096	54,253
Total property, plant and equipment	6,607,552	6,384,396
Construction in progress	209,242	157,922
Total property, plant and equipment and construction in progress	6,816,794	6,542,318
Less accumulated depreciation and amortization	1,668,876	1,749,243
Net property, plant and equipment	$ 5,147,918	$ 4,793,075