Financial Instruments Balance Sheet Location (Details) (USD $)	Mar. 31, 2013	Sep. 30, 2012
Derivatives Fair Value [Line Items]
Derivative Assets Current	$ 7,080,000	$ 24,707,000
Derivative Assets Noncurrent	40,830,000	2,283,000
Derivative Liabilities, Current	59,000	85,381,000
Derivative Liabilities, Noncurrent	0	9,206,000
Derivative Assets Liabilities At Fair Value Net	36,107,000	(91,383,000)
Disposal Group, Including Discontinued Operation, Derivative Assets, Current	200,000	100,000
Disposal Group, Including Discontinued Operation, Derivative Liabilities, Current		300,000
Natural Gas Distribution Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	3,376,000	6,934,000
Derivative Assets Noncurrent	36,582,000	2,283,000
Derivative Liabilities, Current	59,000	85,366,000
Derivative Liabilities, Noncurrent	0	0
Derivative Assets Liabilities At Fair Value Net	40,126,000	(76,260,000)
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
Nonregulated Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	3,704,000	17,773,000
Derivative Assets Noncurrent	4,248,000	0
Derivative Liabilities, Current	0	15,000
Derivative Liabilities, Noncurrent	0	9,206,000
Derivative Assets Liabilities At Fair Value Net	(4,019,000)	(15,123,000)
Designated As Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	27,919,000	(92,602,000)
Designated As Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	10,447,000	19,301,000
Designated As Hedging Instrument [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	37,323,000	1,923,000
Designated As Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Current	17,622,000	108,827,000
Designated As Hedging Instrument [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Noncurrent	2,229,000	4,999,000
Designated As Hedging Instrument [Member] | Natural Gas Distribution Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	36,546,000	(85,040,000)
Designated As Hedging Instrument [Member] | Natural Gas Distribution Segment [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	0	0
Designated As Hedging Instrument [Member] | Natural Gas Distribution Segment [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	36,546,000	0
Designated As Hedging Instrument [Member] | Natural Gas Distribution Segment [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Current	0	85,040,000
Designated As Hedging Instrument [Member] | Natural Gas Distribution Segment [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Noncurrent	0	0
Designated As Hedging Instrument [Member] | Nonregulated Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	(8,627,000)	(7,562,000)
Designated As Hedging Instrument [Member] | Nonregulated Segment [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	10,447,000	19,301,000
Designated As Hedging Instrument [Member] | Nonregulated Segment [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	777,000	1,923,000
Designated As Hedging Instrument [Member] | Nonregulated Segment [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Current	17,622,000	23,787,000
Designated As Hedging Instrument [Member] | Nonregulated Segment [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Noncurrent	2,229,000	4,999,000
Nondesignated [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	8,188,000	1,219,000
Nondesignated [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	80,029,000	105,475,000
Nondesignated [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	52,310,000	63,215,000
Nondesignated [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Current	77,577,000	100,409,000
Nondesignated [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Noncurrent	46,574,000	67,062,000
Nondesignated [Member] | Natural Gas Distribution Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	3,580,000	8,780,000
Nondesignated [Member] | Natural Gas Distribution Segment [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	3,603,000	7,082,000
Nondesignated [Member] | Natural Gas Distribution Segment [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	36,000	2,283,000
Nondesignated [Member] | Natural Gas Distribution Segment [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Current	59,000	585,000
Nondesignated [Member] | Natural Gas Distribution Segment [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Noncurrent	0	0
Nondesignated [Member] | Nonregulated Segment [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Liabilities At Fair Value Net	4,608,000	(7,561,000)
Nondesignated [Member] | Nonregulated Segment [Member] | Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Current	76,426,000	98,393,000
Nondesignated [Member] | Nonregulated Segment [Member] | Deferred Charges And Other Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Assets Noncurrent	52,274,000	60,932,000
Nondesignated [Member] | Nonregulated Segment [Member] | Other Current Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Current	77,518,000	99,824,000
Nondesignated [Member] | Nonregulated Segment [Member] | Deferred Credits And Other Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Liabilities, Noncurrent	$ 46,574,000	$ 67,062,000