Consolidated Statements Of Income And Earnings Reinvested In The Business - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
INCOME
Operating Revenues	$ 1,693,332	$ 1,592,668	$ 1,579,881
Operating Expenses:
Property, Franchise and Other Taxes	88,886	84,393	84,995
Depreciation, Depletion and Amortization	275,660	240,961	224,195
Total Operating Expenses	1,181,523	1,072,945	986,103
Operating Income	511,809	519,723	593,778
Other Income (Expense):
Other Income (Deductions)	(15,542)	(21,174)	(29,777)
Interest Expense on Long-Term Debt	(101,614)	(110,946)	(116,471)
Other Interest Expense	(5,142)	(3,576)	(3,366)
Income Before Income Taxes	389,511	384,027	444,164
Income Tax Expense (Benefit)	85,221	(7,494)	160,682
Net Income Available for Common Stock	304,290	391,521	283,482
EARNINGS REINVESTED IN THE BUSINESS
Balance at Beginning of Year	1,098,900	851,669	676,361
Beginning Retained Earnings Unappropriated And Current Period Net Income	1,403,190	1,243,190	959,843
Dividends on Common Stock	(148,432)	(144,290)	(140,090)
Balance at End of Year	$ 1,272,601	$ 1,098,900	$ 851,669
Earnings Per Common Share, Basic:
Net Income Available for Common Stock (in dollars per share)	$ 3.53	$ 4.56	$ 3.32
Earnings Per Common Share, Diluted:
Net Income Available for Common Stock (in dollars per share)	$ 3.51	$ 4.53	$ 3.30
Weighted Average Number of Shares Outstanding:
Used in Basic Calculation	86,235,550	85,830,597	85,364,929
Used in Diluted Calculation	86,773,259	86,439,698	86,021,386
Utility and Energy Marketing [Member]
INCOME
Operating Revenues	$ 860,985	$ 812,474	$ 755,485
Operating Expenses:
Operation and Maintenance	171,472	168,885	169,731
Exploration and Production and Other [Member]
INCOME
Operating Revenues	636,528	569,808	617,666
Operating Expenses:
Operation and Maintenance	147,457	139,546	141,010
Pipeline and Storage and Gathering [Member]
INCOME
Operating Revenues	195,819	210,386	206,730
Operating Expenses:
Operation and Maintenance	111,783	101,338	90,918
Purchased Gas [Member]
Operating Expenses:
Purchased Gas	386,265	337,822	275,254
Guidance for Recognition and Measurement of Financial Assets and Liabilities [Member]
EARNINGS REINVESTED IN THE BUSINESS
Cumulative Effect of Adoption of Authoritative Guidance	7,437	0	0
Guidance for Reclassification of Stranded Tax Effects [Member]
EARNINGS REINVESTED IN THE BUSINESS
Cumulative Effect of Adoption of Authoritative Guidance	10,406	0	0
Guidance for Stock Based Compensation [Member]
EARNINGS REINVESTED IN THE BUSINESS
Cumulative Effect of Adoption of Authoritative Guidance	$ 0	$ 0	$ 31,916