Financial Instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Sep. 30, 2011
Natural Gas Inventory [Abstract]
Inventory write-down to lower of cost or market			$ 1,700,000
Minimum Length Of Time Hedged In Cash Flow Hedge	one
Maximum Length Of Time Hedged In Cash Flow Hedge	56
Gain Loss On Hedge Ineffectiveness	(6,200,000)	4,100,000	2,200,000	17,500,000
Gain Loss On Derivative Instruments Not Designated Hedges Net Pretax	(12,800,000)	4,000,000	(15,000,000)	8,200,000
Disposal Group, Including Discontinued Operation, Derivative Liabilities, Current	800,000		800,000		1,300,000
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	(29,090,000)	1,279,000	(53,153,000)	3,003,000
Fair value adjustment for natural gas inventory designated as the hedged item	35,087,000	(5,586,000)	50,335,000	(21,211,000)
Total impact on revenue	(5,997,000)	4,307,000	2,818,000	18,208,000
Basis ineffectiveness	(739,000)	(509,000)	102,000	412,000
Timing ineffectiveness	(5,258,000)	4,816,000	2,716,000	17,796,000
Other Comprehensive Income [Abstract]
Treasury lock agreements fair value	15,079,000	6,667,000	13,676,000	38,092,000
Forward commodity contracts fair value	(18,234,000)	(446,000)	(41,912,000)	1,211,000
Treasury lock agreements	317,000	(17,094,000)	633,000	(16,672,000)
Forward commodity contracts	12,919,000	4,471,000	20,022,000	13,164,000
Total other comprehensive income (loss) from hedging, net of tax	10,081,000	(6,402,000)	(7,581,000)	35,795,000
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings			9,018,000
Commodity Contract Gain Loss Reclassified To Earnings			(38,751,000)
Total Hedge Gain Loss Reclassified To Earnings			(29,733,000)
Next Twelve Months [Member]
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings			(1,266,000)
Commodity Contract Gain Loss Reclassified To Earnings			(32,130,000)
Total Hedge Gain Loss Reclassified To Earnings			(33,396,000)
Thereafter [Member]
Expected Earnings [Line Items]
Treasury Lock Gain Loss Reclassified To Earnings			10,284,000
Commodity Contract Gain Loss Reclassified To Earnings			(6,621,000)
Total Hedge Gain Loss Reclassified To Earnings			3,663,000
Natural Gas Distribution Segment [Member]
Commodity Contract Outstanding Volumes [Line Items]
Fair Value	0		0
Cash Flow	0		0
Not designated	6,033		6,033
Total Commodity Contracts Outstanding	6,033		6,033
Derivatives Fair Value [Line Items]
Derivative Assets Current	502,000		502,000		843,000
Derivative Assets Noncurrent	0		0		998,000
Derivative Liabilities Current	47,281,000		47,281,000		11,916,000
Derivative Liabilities Noncurrent	1,000		1,000		67,862,000
Total Financial Instruments	(47,532,000)		(47,532,000)		(79,277,000)
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	502,000	669,000	1,004,000	1,339,000
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income		21,803,000		21,803,000
Total Impact from Cash Flow Hedges	(502,000)	21,134,000	(1,004,000)	20,464,000
Natural Gas Distribution Segment [Member] | Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	(45,818,000)		(45,818,000)		(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		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	(45,818,000)		(45,818,000)		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		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	(1,714,000)		(1,714,000)		(11,750,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	502,000		502,000		843,000
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	0		0		998,000
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(2,215,000)		(2,215,000)		(13,256,000)
Natural Gas Distribution Segment [Member] | Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(1,000)		(1,000)		(335,000)
Nonregulated Segment [Member]
Commodity Contract Outstanding Volumes [Line Items]
Fair Value	(38,340)		(38,340)
Cash Flow	49,098		49,098
Not designated	28,190		28,190
Total Commodity Contracts Outstanding	38,948		38,948
Derivatives Fair Value [Line Items]
Derivative Assets Current	2,877,000		2,877,000		17,501,000
Derivative Assets Noncurrent	10,841,000		10,841,000		0
Derivative Liabilities Current	5,296,000		5,296,000		3,537,000
Derivative Liabilities Noncurrent	5,300,000		5,300,000		10,227,000
Total Financial Instruments	(2,574,000)		(2,574,000)		(25,050,000)
Cash Flow Hedge [Line Items]
Loss reclassified from AOCI into revenue for effective portion of commodity contracts	21,181,000	7,328,000	32,823,000	21,581,000
Loss arising from ineffective portion of commodity contracts	238,000	233,000	668,000	677,000
Total impact on revenue	(21,419,000)	(7,561,000)	(33,491,000)	(22,258,000)
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		0
Total Impact from Cash Flow Hedges	(21,419,000)	(7,561,000)	(33,491,000)	(22,258,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	7,226,000		7,226,000		(16,203,000)
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	77,441,000		77,441,000		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	0		0		174,000
Nonregulated Segment [Member] | Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(59,301,000)		(59,301,000)		(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	(10,914,000)		(10,914,000)		(7,709,000)
Nonregulated Segment [Member] | Nondesignated [Member]
Derivatives Fair Value [Line Items]
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net	(9,800,000)		(9,800,000)		(8,847,000)
Nonregulated Segment [Member] | Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	137,934,000		137,934,000		67,710,000
Nonregulated Segment [Member] | Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	85,951,000		85,951,000		22,379,000
Nonregulated Segment [Member] | Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Current	(164,189,000)		(164,189,000)		(73,865,000)
Nonregulated Segment [Member] | Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities Noncurrent	(69,496,000)		(69,496,000)		(25,071,000)
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		6,000,000
Total Impact from Cash Flow Hedges	$ 0	$ 6,000,000	$ 0	$ 6,000,000