Financial Instruments (Details) (USD $)	3 Months Ended		9 Months Ended
	Jun. 30, 2011 MMcf	Jun. 30, 2010 MMcf	Jun. 30, 2011 MMcf	Jun. 30, 2010 MMcf	Sep. 30, 2010
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	$ (7,837,000)	$ 10,525,000	$ (4,834,000)	$ (20,296,000)
Fair value adjustment for natural gas inventory designated as the hedged item	1,781,000	(14,678,000)	(19,430,000)	(26,195,000)
Total impact on revenue	6,056,000	4,153,000	24,264,000	46,491,000
Basis ineffectiveness	853,000	(235,000)	1,265,000	(684,000)
Timing ineffectiveness	5,203,000	4,388,000	22,999,000	47,175,000
Other Comprehensive Income Abstract
Treasury lock agreements fair value	(8,270,000)	0	29,822,000	0
Forward commodity contracts fair value	(2,668,000)	2,722,000	(1,457,000)	(19,829,000)
Treasury lock agreements	386,000	422,000	(16,286,000)	1,265,000
Forward commodity contracts	2,383,000	5,199,000	15,547,000	24,519,000
Total other comprehensive income (loss) from hedging, net of tax	(8,169,000)	8,343,000	27,626,000	5,955,000
Expected Earnings [Line Items]
Total Hedge Gain Loss Reclassified To Earnings			8,068,000	(8,019,000)
Summary Of Derivative Instruments Abstract
Gain Loss On Hedge Ineffectiveness	5,800,000	3,800,000	23,300,000	44,200,000
Regulated Segments Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate			37.00%
Nonregulated Segment Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate			39.00%
Gain Loss On Derivative Instruments Not Designated Hedges Net Pretax	4,300,000	700,000	3,900,000	13,000,000
Derivative Instruments Gain Loss Reclassification From Accumulated Oci To Income Estimated Net Amount To Be Transferred	12,600,000
Derivative Instruments Gross Gain Loss On Settlement	20,100,000
Natural Gas Distribution Segment [Member]
Commodity Contract Outstanding Volumes [Line Items]
Fair Value	0		0
Cash Flow		0		0
Not designated	16,340		16,340
Total Commodity Contracts Outstanding	16,340		16,340
Derivatives Fair Value [Line Items]
Derivative Assets Current	1,972,000		1,972,000		2,219,000
Derivative Assets Noncurrent	767,000		767,000		47,000
Derivative Liabilities Current	5,207,000		5,207,000		48,942,000
Derivative Liabilities Noncurrent	56,000		56,000		2,924,000
Total Financial Instruments	(2,524,000)		(2,524,000)		(49,600,000)
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into revenue for effective portion of commodity contracts	0	0	0	0
Loss arising from ineffective portion of commodity contracts	0	0	0	0
Total impact on revenue	0	0	0	0
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	614,000	669,000	1,953,000	2,008,000
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income			21,803,000
Total Impact from Cash Flow Hedges	(614,000)	(669,000)	19,850,000	(2,008,000)
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	0		0		0
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	0		0		0
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	0		0		0
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	0		0		0
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	0		0		0
Natural Gas Distribution Segment [Member] | Nondesignated [Member]
Derivatives Fair Value [Line Items]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net	(2,524,000)		(2,524,000)		(49,600,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	1,972,000		1,972,000		2,219,000
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	767,000		767,000		47,000
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(5,207,000)		(5,207,000)		(48,942,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(56,000)		(56,000)		(2,924,000)
Nonregulated Segment [Member]
Commodity Contract Outstanding Volumes [Line Items]
Fair Value	(20,915)		(20,915)
Cash Flow	28,317		28,317
Not designated	18,140		18,140
Total Commodity Contracts Outstanding	25,542		25,542
Derivatives Fair Value [Line Items]
Derivative Assets Current	13,041,000		13,041,000		18,356,000
Derivative Assets Noncurrent	16,000		16,000		890,000
Derivative Liabilities Current	2,995,000		2,995,000		731,000
Derivative Liabilities Noncurrent	6,089,000		6,089,000		6,000,000
Total Financial Instruments	(11,409,000)		(11,409,000)		(12,374,000)
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into revenue for effective portion of commodity contracts	3,907,000	8,523,000	25,488,000	40,196,000
Loss arising from ineffective portion of commodity contracts	281,000	350,000	958,000	2,307,000
Total impact on revenue	(4,188,000)	(8,873,000)	(26,446,000)	(42,503,000)
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	0	0	0	0
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income			0
Total Impact from Cash Flow Hedges	(4,188,000)	(8,873,000)	(26,446,000)	(42,503,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	(10,397,000)		(10,397,000)		(23,306,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	11,529,000		11,529,000		40,030,000
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	241,000		241,000		2,461,000
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(15,930,000)		(15,930,000)		(56,575,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(6,237,000)		(6,237,000)		(9,222,000)
Nonregulated Segment [Member] | Nondesignated [Member]
Derivatives Fair Value [Line Items]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net	(1,012,000)		(1,012,000)		10,932,000
Nonregulated Segment [Member] | Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	19,174,000		19,174,000		16,459,000
Nonregulated Segment [Member] | Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	7,093,000		7,093,000		2,056,000
Nonregulated Segment [Member] | Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(20,109,000)		(20,109,000)		(7,178,000)
Nonregulated Segment [Member] | Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(7,170,000)		(7,170,000)		(405,000)
Regulated Transmission and Storage Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	0		0		0
Derivative Assets Noncurrent	0		0		0
Derivative Liabilities Current	0		0		0
Derivative Liabilities Noncurrent	0		0		0
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into revenue for effective portion of commodity contracts	0	0	0	0
Loss arising from ineffective portion of commodity contracts	0	0	0	0
Total impact on revenue	0	0	0	0
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	0	0	0	0
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income			6,000,000
Total Impact from Cash Flow Hedges	0	0	6,000,000	0
Next Twelve Months [Member]
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings			(1,266,000)
Total Hedge Gain Loss Reclassified To Earnings			(3,905,000)
Thereafter [Member]
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings			9,334,000
Total Hedge Gain Loss Reclassified To Earnings			$ (4,114,000)