Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended	6 Months Ended		12 Months Ended		6 Months Ended	12 Months Ended	6 Months Ended		12 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended					3 Months Ended		3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Sep. 30, 2011	Mar. 31, 2012 Exploration And Production [Member]	Mar. 31, 2011 Exploration And Production [Member]	Sep. 30, 2011 Exploration And Production [Member]	Sep. 30, 2010 Exploration And Production [Member]	Mar. 31, 2012 All Other [Member]	Sep. 30, 2011 All Other [Member]	Mar. 31, 2012 Pipeline And Storage [Member]	Mar. 31, 2011 Pipeline And Storage [Member]	Sep. 30, 2011 Pipeline And Storage [Member]	Mar. 31, 2012 Utility [Member]	Mar. 31, 2012 Stock Options [Member]	Mar. 31, 2012 Stock Options [Member]	Mar. 31, 2012 Restricted Share Awards [Member]	Mar. 31, 2012 Restricted Share Awards [Member]	Mar. 31, 2012 Restricted Stock Units [Member]	Mar. 31, 2012 Restricted Stock Units [Member]	Mar. 31, 2012 Nonperformance Based Stock Appreciation Right [Member]	Mar. 31, 2012 Nonperformance Based Stock Appreciation Right [Member]	Mar. 31, 2012 Exchange-Traded Futures Contracts [Member]	Sep. 30, 2011 Exchange-Traded Futures Contracts [Member]	Mar. 31, 2012 Over-The-Counter Crude Oil Swap Agreements [Member]	Sep. 30, 2011 Over-The-Counter Crude Oil Swap Agreements [Member]	Mar. 31, 2011 Reclassification Of Regulatory Liabilities To Regulatory Assets Due To From Gross To Net Methodology [Member]	Sep. 30, 2011 Reclassification Of Regulatory Liabilities To Regulatory Assets Due To From Gross To Net Methodology [Member]	Mar. 31, 2011 Sale Of Equity Method Investee [Member]
Summary Of Significant Accounting Policies [Line Items]
Equity Method Investment, Ownership Percentage		50.00%		50.00%
Sale of equity method investment			$ 0	$ 59,365,000																										$ 59,400,000
Gain on sale of equity method investment	0	50,879,000	0	50,879,000																										50,900,000
Regulatory Assets																													63,700,000
Change In Other Liabilities			44,323,000	(23,214,000)																								500,000
Change In Other Assets			48,692,000	(15,239,000)																								(500,000)
Accrued capital expenditure, non-cash investing activity						93,600,000	43,900,000	63,500,000	55,500,000	7,900,000	1,400,000	12,900,000	2,000,000	7,300,000
Hedging collateral deposits	18,872,000		18,872,000		19,701,000																			10,100,000	5,500,000	8,800,000	14,200,000
Gas stored underground - current	16,195,000		16,195,000		54,325,000										52,100,000
Capitalized costs of unproved properties excluded from amortization			275,200,000		226,300,000
Full cost ceiling test discount factor	10.00%		10.00%
Amount full cost ceiling exceeds book value of oil and gas properties	279,600,000		279,600,000
Antidilutive securities	879,847	10,959	297,081	140
Stock options granted																0	0
Share based compensation other than options grants in period																		0	41,525	1,500	57,500	0	166,000
Share based compensation arrangement by share based payment award other than options grants in period weighted average grant date excise price																							$ 55.09
Granted in fiscal year, weighted average grant date fair value																			$ 55.09	$ 43.84	$ 48.64		$ 11.20
Tax Benefit Total Not Realized Due To Tax Loss Carryforwards	$ 32,600,000		$ 32,600,000		$ 19,500,000