Financial Instruments Fair Value Hedges and Cash Flow Hedges (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Effect of Fair Value Hedges on Results of Operations [Abstract]
Commodity contracts	$ 3,587	$ (17,846)	$ (4,974)	$ (10,532)
Fair value adjustment for natural gas inventory designated as the hedged item	(7,450)	19,586	6,329	28,405
Total (increase) decrease to purchased gas cost	(3,863)	1,740	1,355	17,873
Basis ineffectiveness	(579)	1,458	(1,199)	1,218
Timing ineffectiveness	(3,284)	282	2,554	16,655
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	7,184	(5,199)	4,574	(10,359)
Gain (loss) arising from ineffective portion of commodity contracts	142	(83)	24	(102)
Total impact on purchased gas cost	7,326	(5,282)	4,598	(10,461)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(1,057)	(873)	(2,115)	(1,375)
Total Impact from Cash Flow Hedges	6,269	(6,155)	2,483	(11,836)
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	0
Gain (loss) arising from ineffective portion of commodity contracts	0	0	0	0
Total impact on purchased gas cost	0	0	0	0
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	(1,057)	(873)	(2,115)	(1,375)
Total Impact from Cash Flow Hedges	(1,057)	(873)	(2,115)	(1,375)
Nonregulated Segment [Member]
Cash Flow Hedge [Line Items]
Gain (loss) reclassified from AOCI into purchased gas cost for effective portion of commodity contracts	7,184	(5,199)	4,574	(10,359)
Gain (loss) arising from ineffective portion of commodity contracts	142	(83)	24	(102)
Total impact on purchased gas cost	7,326	(5,282)	4,598	(10,461)
Loss on settled Treasury Lock agreements reclassified from AOCI into interest expense	0	0	0	0
Total Impact from Cash Flow Hedges	$ 7,326	$ (5,282)	$ 4,598	$ (10,461)