Consolidated Statements Of Income And Earnings Reinvested In The Business - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
INCOME
Operating Revenues	$ 1,579,881	$ 1,452,416	$ 1,760,913
Operating Expenses:
Purchased Gas	275,254	147,982	349,984
Property, Franchise and Other Taxes	84,995	81,714	89,564
Depreciation, Depletion and Amortization	224,195	249,417	336,158
Impairment of Oil and Gas Producing Properties	0	948,307	1,126,257
Total Operating Expenses	1,027,036	1,868,934	2,371,966
Operating Income (Loss)	552,845	(416,518)	(611,053)
Other Income (Expense):
Other Income	7,043	9,820	8,039
Interest Income	4,113	4,235	3,922
Interest Expense on Long-Term Debt	(116,471)	(117,347)	(95,916)
Other Interest Expense	(3,366)	(3,697)	(3,555)
Income (Loss) Before Income Taxes	444,164	(523,507)	(698,563)
Income Tax Expense (Benefit)	160,682	(232,549)	(319,136)
Net Income (Loss) Available for Common Stock	283,482	(290,958)	(379,427)
EARNINGS REINVESTED IN THE BUSINESS
Balance at Beginning of Year	676,361	1,103,200	1,614,361
Beginning Retained Earnings Unappropriated And Current Period Net Income Loss	959,843	812,242	1,234,934
Dividends on Common Stock	(140,090)	(135,881)	(131,734)
Cumulative Effect of Adoption of Authoritative Guidance for Stock-Based Compensation	31,916	0	0
Balance at End of Year	$ 851,669	$ 676,361	$ 1,103,200
Earnings Per Common Share, Basic:
Net Income (Loss) Available for Common Stock (in usd per share)	$ 3.32	$ (3.43)	$ (4.50)
Earnings Per Common Share, Diluted:
Net Income (Loss) Available for Common Stock (in usd per share)	$ 3.30	$ (3.43)	$ (4.50)
Weighted Average Number of Shares Outstanding:
Used in Basic Calculation	85,364,929	84,847,993	84,387,755
Used in Diluted Calculation	86,021,386	84,847,993	84,387,755
Utility and Energy Marketing [Member]
INCOME
Operating Revenues	$ 755,485	$ 624,602	$ 860,618
Operating Expenses:
Operation and Maintenance	199,293	192,512	203,249
Exploration and Production and Other [Member]
INCOME
Operating Revenues	617,666	611,766	696,709
Operating Expenses:
Operation and Maintenance	145,099	160,201	184,024
Pipeline and Storage and Gathering [Member]
INCOME
Operating Revenues	206,730	216,048	203,586
Operating Expenses:
Operation and Maintenance	$ 98,200	$ 88,801	$ 82,730