Fair Value Measurements (Details) (USD $)	Mar. 31, 2012	Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value	$ 65,419,000	$ 52,633,000
Fair Value [Abstract]
Cash Collateral Right To Reclaim Cash	5,700,000	28,800,000
Debt Instrument Fair Value	2,583,071,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	2,800,000	12,400,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	2,900,000	16,400,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	52,013,000	15,262,000
Total financial instruments	52,013,000	15,262,000
Hedged portion of gas stored underground	73,043,000	47,940,000
Available For Sale Securities Equity Securities	38,424,000	36,444,000
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	38,424,000	36,444,000
Total assets	163,480,000	99,646,000
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	76,476,000	22,091,000
Total liabilities	76,476,000	22,091,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	502,000	1,841,000
Nonregulated Segement Derivative Assets	249,313,000	97,396,000
Total financial instruments	249,815,000	99,237,000
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	23,637,000	14,366,000
Available-for-sale securities money market funds	3,358,000	1,823,000
Fair value	26,995,000	16,189,000
Total assets	276,810,000	115,426,000
Natural gas distribution segment	48,034,000	81,118,000
Nonregulated Segement Derivative Liabilities	227,424,000	115,617,000
Total liabilities	275,458,000	196,735,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	0	0
Total financial instruments	0	0
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	0	0
Total assets	0	0
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	0	0
Total liabilities	0	0
Netting And Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Assets	(287,608,000)	(95,156,000)
Total financial instruments	(287,608,000)	(95,156,000)
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	0	0
Total assets	(287,608,000)	(95,156,000)
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	(293,304,000)	(123,943,000)
Total liabilities	$ (293,304,000)	$ (123,943,000)