Fair Value Measurements (Details) (USD $)	Dec. 31, 2012	Sep. 30, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	$ 13,383,000	$ 9,365,000
Nonregulated Segment Derivative Assets	17,857,000	17,773,000
Total financial instruments	31,240,000	27,138,000
Hedged portion of gas stored underground	87,401,000	67,192,000
Available For Sale Securities Equity Securities	39,499,000	40,212,000
Available For Sale Securities Debt Securities	23,565,000	22,552,000
Available-for-sale securities money market funds	801,000	1,634,000
Fair value	63,865,000	64,398,000
Total assets	182,506,000	158,728,000
Natural gas distribution segment	77,580,000	85,625,000
Nonregulated Segment Derivative Liabilities	2,860,000	9,221,000
Total liabilities	80,440,000	94,846,000
Fair Value [Abstract]
Cash Collateral Right To Reclaim Cash	16,600,000	23,700,000
Debt Instrument Fair Value	2,403,501,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	0	5,900,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	16,600,000	17,800,000
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 Segment Derivative Assets	1,043,000	714,000
Total financial instruments	1,043,000	714,000
Hedged portion of gas stored underground	87,401,000	67,192,000
Available For Sale Securities Equity Securities	39,499,000	40,212,000
Available For Sale Securities Debt Securities	0	0
Available-for-sale securities money market funds	0	0
Fair value	39,499,000	40,212,000
Total assets	127,943,000	108,118,000
Natural gas distribution segment	0	0
Nonregulated Segment Derivative Liabilities	1,261,000	4,563,000
Total liabilities	1,261,000	4,563,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Natural gas distribution segment	13,383,000	9,365,000
Nonregulated Segment Derivative Assets	173,718,000	179,835,000
Total financial instruments	187,101,000	189,200,000
Hedged portion of gas stored underground	0	0
Available For Sale Securities Equity Securities	0	0
Available For Sale Securities Debt Securities	23,565,000	22,552,000
Available-for-sale securities money market funds	801,000	1,634,000
Fair value	24,366,000	24,186,000
Total assets	211,467,000	213,386,000
Natural gas distribution segment	77,580,000	85,625,000
Nonregulated Segment Derivative Liabilities	175,062,000	191,109,000
Total liabilities	252,642,000	276,734,000
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 Segment 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 Segment 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 Segment Derivative Assets	(156,904,000)	(162,776,000)
Total financial instruments	(156,904,000)	(162,776,000)
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	(156,904,000)	(162,776,000)
Natural gas distribution segment	0	0
Nonregulated Segment Derivative Liabilities	(173,463,000)	(186,451,000)
Total liabilities	$ (173,463,000)	$ (186,451,000)