Consolidated Statements Of Income And Earnings Reinvested In The Business (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
INCOME
Operating Revenues	$ 419,655	$ 422,500
Operating Expenses:
Purchased Gas	94,034	70,243
Property, Franchise and Other Taxes	20,848	20,379
Depreciation, Depletion and Amortization	55,830	56,196
Total Operating Expenses	277,660	250,361
Operating Income	141,995	172,139
Other Income (Expense):
Interest Income	2,249	1,600
Other Income	1,722	1,614
Interest Expense on Long-Term Debt	(28,087)	(29,103)
Other Interest Expense	(502)	(910)
Income Before Income Taxes	117,377	145,340
Income Tax Expense (Benefit)	(81,277)	56,432
Net Income Available for Common Stock	198,654	88,908
EARNINGS REINVESTED IN THE BUSINESS
Balance at Beginning of Period	851,669	676,361
Beginning Retained Earnings Unappropriated And Current Period Net Income	1,050,323	765,269
Dividends on Common Stock	(35,590)	(34,544)
Cumulative Effect of Adoption of Authoritative Guidance for Stock-Based Compensation	0	31,916
Balance at December 31	$ 1,014,733	$ 762,641
Earnings Per Common Share, Basic:
Net Income Available for Common Stock (in dollars per share)	$ 2.32	$ 1.04
Earnings Per Common Share, Diluted:
Net Income Available for Common Stock (in dollars per share)	$ 2.30	$ 1.04
Weighted Average Common Shares Outstanding:
Used in Basic Calculation (shares)	85,630,296	85,189,851
Used in Diluted Calculation (shares)	86,325,537	85,797,989
Dividends Per Common Share:
Dividends Declared (in dollars per share)	$ 0.415	$ 0.405
Utility and Energy Marketing [Member]
INCOME
Operating Revenues	$ 225,725	$ 207,780
Operating Expenses:
Operation and Maintenance	51,369	50,422
Exploration and Production and Other [Member]
INCOME
Operating Revenues	140,450	161,694
Operating Expenses:
Operation and Maintenance	35,542	30,461
Pipeline and Storage and Gathering [Member]
INCOME
Operating Revenues	53,480	53,026
Operating Expenses:
Operation and Maintenance	$ 20,037	$ 22,660