Consolidated Statements Of Income And Earnings Reinvested In The Business (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
INCOME
Operating Revenues	$ 540,905	$ 522,075	$ 960,561	$ 944,576
Operating Expenses:
Purchased Gas	176,608	147,971	270,642	218,214
Property, Franchise and Other Taxes	22,802	22,542	43,650	42,921
Depreciation, Depletion and Amortization	61,155	56,999	116,985	113,194
Total Operating Expenses	384,203	352,118	661,863	602,479
Operating Income	156,702	169,957	298,698	342,097
Other Income (Expense):
Interest Income	1,025	391	3,275	1,991
Other Income	770	1,744	2,492	3,356
Interest Expense on Long-Term Debt	(27,148)	(28,913)	(55,235)	(58,016)
Other Interest Expense	(1,233)	(924)	(1,736)	(1,834)
Income Before Income Taxes	130,116	142,255	247,494	287,594
Income Tax Expense (Benefit)	38,269	52,971	(43,007)	109,403
Net Income Available for Common Stock	91,847	89,284	290,501	178,191
EARNINGS REINVESTED IN THE BUSINESS
Balance at Beginning of Period	1,014,733	762,641	851,669	676,361
Beginning Retained Earnings Unappropriated And Current Period Net Income	1,106,580	851,925	1,142,170	854,552
Dividends on Common Stock	(35,641)	(34,577)	(71,231)	(69,120)
Cumulative Effect of Adoption of Authoritative Guidance for Stock-Based Compensation	0	0	0	31,916
Balance at March 31	$ 1,070,939	$ 817,348	$ 1,070,939	$ 817,348
Earnings Per Common Share, Basic:
Net Income Available for Common Stock (in dollars per share)	$ 1.07	$ 1.05	$ 3.39	$ 2.09
Earnings Per Common Share, Diluted:
Net Income Available for Common Stock (in dollars per share)	$ 1.06	$ 1.04	$ 3.37	$ 2.07
Weighted Average Common Shares Outstanding:
Used in Basic Calculation (shares)	85,809,233	85,334,887	85,718,779	85,261,575
Used in Diluted Calculation (shares)	86,323,636	86,006,614	86,318,892	85,897,282
Dividends Per Common Share:
Dividends Declared (in dollars per share)	$ 0.415	$ 0.405	$ 0.830	$ 0.810
Utility and Energy Marketing [Member]
INCOME
Operating Revenues	$ 339,422	$ 308,889	$ 565,147	$ 516,669
Operating Expenses:
Operation and Maintenance	61,410	63,907	112,780	114,329
Exploration and Production and Other [Member]
INCOME
Operating Revenues	147,868	159,997	288,318	321,691
Operating Expenses:
Operation and Maintenance	39,586	37,593	75,127	68,055
Pipeline and Storage and Gathering [Member]
INCOME
Operating Revenues	53,615	53,189	107,096	106,216
Operating Expenses:
Operation and Maintenance	$ 22,642	$ 23,106	$ 42,679	$ 45,766