Financial Instruments (Details) (USD $)	12 Months Ended
	Sep. 30, 2012 Bcf	Sep. 30, 2011	Sep. 30, 2010
Derivatives Fair Value [Line Items]
Derivative Assets Current	$ 24,707,000	$ 18,344,000
Derivative Assets Noncurrent	2,283,000	998,000
Derivative Liabilities Current	85,381,000	15,453,000
Derivative Liabilities Noncurrent	9,206,000	78,089,000
Total Financial Instruments	(91,383,000)	(104,327,000)
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	(30,266,000)	(16,552,000)	(34,650,000)
Fair value adjustment for natural gas inventory designated as the hedged item	5,797,000	(9,824,000)	(19,867,000)
Total impact on purchased gas cost	24,469,000	26,376,000	54,517,000
Basis ineffectiveness	1,170,000	803,000	(1,272,000)
Timing ineffectiveness	23,299,000	25,573,000	55,789,000
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into purchased gas for effective portion of commodity contracts	(62,678,000)	(28,430,000)	(44,809,000)
Loss arising from ineffective portion of commodity contracts	(1,369,000)	(1,585,000)	(2,717,000)
Total impact on purchased gas cost	(64,047,000)	(30,015,000)	(47,526,000)
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	(2,009,000)	(2,455,000)	(2,678,000)
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income		27,803,000
Total Impact from Cash Flow Hedges	(66,056,000)	(4,667,000)	(50,204,000)
Other Comprehensive Income [Abstract]
Treasury lock agreements fair value	(11,458,000)	(12,720,000)
Forward commodity contracts fair value	(30,366,000)	(12,096,000)
Treasury lock agreements	1,342,000	(15,969,000)
Forward commodity contracts	38,232,000	17,344,000
Total other comprehensive income (loss) from hedging, net of tax	(2,250,000)	(23,441,000)
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	9,727,000
Total Hedge Gain Loss Reclassified To Earnings	732,000
Commodity Contract Gain Loss Reclassified To Earnings	(8,995,000)
Natural Gas Inventory [Abstract]
Inventory Write-down	1,700,000
Summary Of Derivative Instruments Abstract
Minimum Length Of Time Hedged In Cash Flow Hedge	one
Maximum Length Of Time Hedged In Cash Flow Hedge	63 months
Net Open Positions	0.4
Gain Loss On Hedge Ineffectiveness	23,100,000	24,800,000	51,800,000
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	(2,500,000)	(1,400,000)	15,400,000
Disposal Group, Including Discontinued Operation, Derivative Liabilities, Current	300,000	1,300,000
Disposal Group, Including Discontinued Operation, Derivative Assets, Current	100,000
Gain (Loss) on Treasury Lock Agreements Settlements, Net of Tax		12,600,000
One Year From Balance Sheet Date Member
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	(1,276,000)
Total Hedge Gain Loss Reclassified To Earnings	(8,447,000)
Commodity Contract Gain Loss Reclassified To Earnings	(7,171,000)
More Than One And Within Two Years From Balance Sheet Date Member
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	(1,276,000)
Total Hedge Gain Loss Reclassified To Earnings	(3,184,000)
Commodity Contract Gain Loss Reclassified To Earnings	(1,908,000)
More Than Two And Within Three Years From Balance Sheet Date Member
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	606,000
Total Hedge Gain Loss Reclassified To Earnings	616,000
Commodity Contract Gain Loss Reclassified To Earnings	10,000
More Than Three And Within Four Years From Balance Sheet Date Member
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	776,000
Total Hedge Gain Loss Reclassified To Earnings	822,000
Commodity Contract Gain Loss Reclassified To Earnings	46,000
More Than Four And Within Five Years From Balance Sheet Date Member
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	675,000
Total Hedge Gain Loss Reclassified To Earnings	703,000
Commodity Contract Gain Loss Reclassified To Earnings	28,000
More Than Five Years From Balance Sheet Date And Thereafter Member
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings	10,222,000
Total Hedge Gain Loss Reclassified To Earnings	10,222,000
Commodity Contract Gain Loss Reclassified To Earnings	0
Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Net assets liabilities	(92,602,000)	(83,730,000)
Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	19,301,000	22,396,000
Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	1,923,000	174,000
Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(108,827,000)	(31,064,000)
Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(4,999,000)	(75,236,000)
Nondesignated [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(67,062,000)
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net	1,219,000	(20,597,000)
Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	105,475,000	68,553,000
Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	63,215,000	23,377,000
Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(100,409,000)	(87,121,000)
Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent		(25,406,000)
Natural Gas Distribution Segment [Member]
Commodity Contract Outstanding Volumes [Line Items]
Fair Value	0
Cash Flow	0
Not designated	24,185
Total Commodity Contracts Outstanding	24,185
Derivatives Fair Value [Line Items]
Derivative Assets Current	6,934,000	843,000
Derivative Assets Noncurrent	2,283,000	998,000
Derivative Liabilities Current	85,366,000	11,916,000
Derivative Liabilities Noncurrent	0	67,862,000
Total Financial Instruments	(76,260,000)	(79,277,000)
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into purchased gas for effective portion of commodity contracts	0	0	0
Loss arising from ineffective portion of commodity contracts	0	0	0
Total impact on purchased gas cost	0	0	0
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	(2,009,000)	(2,455,000)	(2,678,000)
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income		21,803,000
Total Impact from Cash Flow Hedges	(2,009,000)	19,348,000	(2,678,000)
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Net assets liabilities	(85,040,000)	(67,527,000)
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	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
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(85,040,000)	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	(67,527,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member]
Derivatives Fair Value [Line Items]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net	8,780,000	(11,750,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	7,082,000	843,000
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	2,283,000	998,000
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(585,000)	(13,256,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	0	(335,000)
Nonregulated Segment [Member]
Commodity Contract Outstanding Volumes [Line Items]
Fair Value	(22,650)
Cash Flow	35,300
Not designated	49,155
Total Commodity Contracts Outstanding	61,805
Derivatives Fair Value [Line Items]
Derivative Assets Current	17,773,000	17,501,000
Derivative Assets Noncurrent	0	0
Derivative Liabilities Current	15,000	3,537,000
Derivative Liabilities Noncurrent	9,206,000	10,227,000
Total Financial Instruments	(15,123,000)	(25,050,000)
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into purchased gas for effective portion of commodity contracts	(62,678,000)	(28,430,000)	(44,809,000)
Loss arising from ineffective portion of commodity contracts	(1,369,000)	(1,585,000)	(2,717,000)
Total impact on purchased gas cost	(64,047,000)	(30,015,000)	(47,526,000)
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	0	0	0
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income		0
Total Impact from Cash Flow Hedges	(64,047,000)	(30,015,000)	(47,526,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Net assets liabilities	(7,562,000)	(16,203,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	19,301,000	22,396,000
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	1,923,000	174,000
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(23,787,000)	(31,064,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(4,999,000)	(7,709,000)
Nonregulated Segment [Member] | Nondesignated [Member]
Derivatives Fair Value [Line Items]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net	(7,561,000)	(8,847,000)
Nonregulated Segment [Member] | Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	98,393,000	67,710,000
Nonregulated Segment [Member] | Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	60,932,000	22,379,000
Nonregulated Segment [Member] | Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(99,824,000)	(73,865,000)
Nonregulated Segment [Member] | Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(67,062,000)	(25,071,000)
Regulated Transmission and Storage Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	0	0
Derivative Assets Noncurrent	0	0
Derivative Liabilities Current	0	0
Derivative Liabilities Noncurrent	0	0
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into purchased gas for effective portion of commodity contracts	0	0	0
Loss arising from ineffective portion of commodity contracts	0	0	0
Total impact on purchased gas cost	0	0	0
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	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	$ 6,000,000	$ 0