Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Operating Activities
Net Income (Loss) Available for Common Stock	$ 283,482	$ (290,958)	$ (379,427)
Adjustments to Reconcile Net Income (Loss) to Net Cash Provided by Operating Activities:
Impairment of Oil and Gas Producing Properties	0	948,307	1,126,257
Depreciation, Depletion and Amortization	224,195	249,417	336,158
Deferred Income Taxes	117,975	(246,794)	(357,587)
Excess Tax Benefits Associated with Stock-Based Compensation Awards	0	(1,868)	(9,064)
Stock-Based Compensation	12,262	5,755	3,208
Other	16,476	12,620	9,823
Change in:
Hedging Collateral Deposits	(257)	9,640	(8,390)
Receivables and Unbilled Revenue	(3,380)	(6,408)	51,638
Gas Stored Underground and Materials and Supplies	(1,417)	(3,532)	3,438
Unrecovered Purchased Gas Costs	(2,183)	(2,440)	0
Other Current Assets	7,849	3,179	3,150
Accounts Payable	17,192	(40,664)	34,687
Amounts Payable to Customers	(19,537)	(37,241)	23,033
Customer Advances	939	(1,474)	(2,769)
Customer Security Deposits	4,353	(471)	729
Other Accruals and Current Liabilities	27,004	3,453	(7,173)
Other Assets	(2,885)	1,941	2,696
Other Liabilities	2,183	(13,483)	23,173
Net Cash Provided by Operating Activities	684,251	588,979	853,580
Investing Activities
Capital Expenditures	(450,335)	(581,576)	(1,018,179)
Net Proceeds from Sale of Oil and Gas Producing Properties	26,554	137,316	0
Other	1,216	(9,236)	(6,611)
Net Cash Used in Investing Activities	(422,565)	(453,496)	(1,024,790)
Financing Activities
Change in Notes Payable to Banks and Commercial Paper	0	0	(85,600)
Excess Tax Benefits Associated with Stock-Based Compensation Awards	0	1,868	9,064
Net Proceeds from Issuance of Long-Term Debt	295,151	0	444,635
Net Proceeds from Issuance of Common Stock	7,784	13,849	10,540
Dividends Paid on Common Stock	(139,063)	(134,824)	(130,719)
Net Cash Provided by (Used in) Financing Activities	163,872	(119,107)	247,920
Net Increase in Cash and Temporary Cash Investments	425,558	16,376	76,710
Cash and Temporary Cash Investments At Beginning of Year	129,972	113,596	36,886
Cash and Temporary Cash Investments At End of Year	555,530	129,972	113,596
Supplemental Disclosure of Cash Flow Information
Cash Paid for Interest	116,894	119,563	90,747
Cash Paid for Income Taxes	34,826	34,240	18,657
Supplemental Disclosure of Cash Flow Information, Non-Cash Investing Activities
Non-Cash Capital Expenditures	72,216	60,434	118,959
Receivable from Sale of Oil and Gas Producing Properties	$ 0	$ 19,543	$ 0