Fair Value Measurements (Details) (USD $)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	$ 9,365,000	$ 1,841,000
Nonregulated Segement Derivative Assets	17,773,000	17,502,000
Total financial instruments	27,138,000	19,343,000
Hedged portion of gas stored underground	67,192,000	47,940,000
Available For Sale Securities, Registered Investment Companies	40,212,000	36,444,000
Fair value	64,398,000	52,633,000
Total assets	158,728,000	119,916,000
Natural gas distribution segment	85,625,000	81,118,000
Nonregulated Segement Derivative Liabilities	9,221,000	13,765,000
Total liabilities	94,846,000	94,883,000
Fair Value [Abstract]
Cash Collateral Right To Reclaim Cash	23,700,000	28,800,000
Debt Instrument Fair Value	2,426,434,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	5,900,000	12,400,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	17,800,000	16,400,000
Other Matters [Abstract]
Asset Impairment Charges Park City	5,300,000	11,000,000
Asset Impairment Charges Fort Necessity		19,300,000
Nonrecurring fair value measurement	500,000
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	714,000	8,502,000
Total financial instruments	714,000	8,502,000
Hedged portion of gas stored underground	67,192,000	47,940,000
Available-for-sale Securities, Money Market Funds	0	0
Available For Sale Securities, Registered Investment Companies	40,212,000	36,444,000
Available For Sale Securities, Bonds	0
Fair value	40,212,000	36,444,000
Total assets	108,118,000	92,886,000
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	4,563,000	9,324,000
Total liabilities	4,563,000	9,324,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	9,365,000	1,841,000
Nonregulated Segement Derivative Assets	179,835,000	104,156,000
Total financial instruments	189,200,000	105,997,000
Hedged portion of gas stored underground	0	0
Available-for-sale Securities, Money Market Funds	1,634,000	1,823,000
Available For Sale Securities, Registered Investment Companies	0	0
Available For Sale Securities, Bonds	22,552,000	14,366,000
Fair value	24,186,000	16,189,000
Total assets	213,386,000	122,186,000
Natural gas distribution segment	85,625,000	81,118,000
Nonregulated Segement Derivative Liabilities	191,109,000	128,384,000
Total liabilities	276,734,000	209,502,000
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, Money Market Funds	0	0
Available For Sale Securities, Registered Investment Companies	0	0
Available For Sale Securities, Bonds	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	(162,776,000)	(95,156,000)
Total financial instruments	(162,776,000)	(95,156,000)
Hedged portion of gas stored underground	0	0
Available-for-sale Securities, Money Market Funds	0	0
Available For Sale Securities, Registered Investment Companies	0	0
Available For Sale Securities, Bonds	0
Fair value	0	0
Total assets	(162,776,000)	(95,156,000)
Natural gas distribution segment	0	0
Nonregulated Segement Derivative Liabilities	(186,451,000)	(123,943,000)
Total liabilities	$ (186,451,000)	$ (123,943,000)