Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	$ 2,165	$ 30,266	$ 16,552
Fair value adjustment for natural gas inventory designated as the hedged item	15,938	(5,797)	9,824
Total (increase) decrease in purchased gas cost	18,103	24,469	26,376
Fair Value Hedge Basis Ineffectiveness	(208)	1,170	803
Fair Value Hedge Timing Ineffectiveness	18,311	23,299	25,573
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(10,778)	(62,678)	(28,430)
Gain (loss) arising from ineffective portion of commodity contracts	97	(1,369)	(1,585)
Total impact on purchased gas cost	(10,681)	(64,047)	(30,015)
Net loss on settled Treasury lock agreements reclassified from AOCI into interest expense	(3,489)	(2,009)	(2,455)
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income			27,803
Total Impact from Cash Flow Hedges	(14,170)	(66,056)	(4,667)
Natural Gas Distribution Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	0	0	0
Gain (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	(3,489)	(2,009)	(2,455)
Gain On Unwinding Of Treasury Lock Reclassified From Accumulated Oci To Miscellaneous Income			21,803
Total Impact from Cash Flow Hedges	(3,489)	(2,009)	19,348
Regulated Transmission and Storage Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	0	0	0
Gain (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
Total Impact from Cash Flow Hedges	0	0	6,000
Nonregulated Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	(10,778)	(62,678)	(28,430)
Gain (loss) arising from ineffective portion of commodity contracts	97	(1,369)	(1,585)
Total impact on purchased gas cost	(10,681)	(64,047)	(30,015)
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	$ (10,681)	$ (64,047)	$ (30,015)