Fair Value Measurements (Details) (USD $)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	$ 27,138,000		$ 127,453,000
Hedged portion of gas stored underground	67,192,000		44,758,000
Available-for-sale Securities	64,398,000		72,682,000
Total assets	158,728,000		244,893,000
Financial instruments, liabilities	94,846,000		7,676,000
Fair Value [Abstract]
Cash Collateral Right To Reclaim Cash	23,700,000		24,800,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	5,900,000		8,600,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	17,800,000		16,200,000
Debt Instrument Carrying Amount	1,960,131,000		2,460,000,000
Debt Instrument Fair Value			2,676,487,000
Other Matters [Abstract]
Asset Impairment Charges Park City	5,300,000	11,000,000
Nonrecurring fair value measurement	500,000
Asset Impairment Charges Fort Necessity		19,300,000
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,634,000		4,428,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	40,212,000		40,094,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	22,552,000		28,160,000
Natural Gas Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	9,365,000		111,191,000
Financial instruments, liabilities	85,625,000		1,543,000
Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	17,773,000		16,262,000
Financial instruments, liabilities	9,221,000		6,133,000
Fair Value [Abstract]
Cash Collateral Right To Reclaim Cash	23,700,000		24,800,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	5,900,000		8,600,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	17,800,000		16,200,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	714,000		745,000
Hedged portion of gas stored underground	67,192,000		44,758,000
Available-for-sale Securities	40,212,000		40,094,000
Total assets	108,118,000		85,597,000
Financial instruments, liabilities	4,563,000		158,000
Fair Value Inputs Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Fair Value Inputs Level 1 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	40,212,000		40,094,000
Fair Value Inputs Level 1 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Fair Value Inputs Level 1 [Member] | Natural Gas Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	0		0
Financial instruments, liabilities	0		0
Fair Value Inputs Level 1 [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	714,000		745,000
Financial instruments, liabilities	4,563,000		158,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	189,200,000		226,326,000
Hedged portion of gas stored underground	0		0
Available-for-sale Securities	24,186,000		32,588,000
Total assets	213,386,000		258,914,000
Financial instruments, liabilities	276,734,000		131,965,000
Fair Value Inputs Level 2 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,634,000		4,428,000
Fair Value Inputs Level 2 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Fair Value Inputs Level 2 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	22,552,000		28,160,000
Fair Value Inputs Level 2 [Member] | Natural Gas Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	9,365,000		111,191,000
Financial instruments, liabilities	85,625,000		1,543,000
Fair Value Inputs Level 2 [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	179,835,000		115,135,000
Financial instruments, liabilities	191,109,000		130,422,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	0		0
Hedged portion of gas stored underground	0		0
Available-for-sale Securities	0		0
Total assets	0		0
Financial instruments, liabilities	0		0
Fair Value Inputs Level 3 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Fair Value Inputs Level 3 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Fair Value Inputs Level 3 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Fair Value Inputs Level 3 [Member] | Natural Gas Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	0		0
Financial instruments, liabilities	0		0
Fair Value Inputs Level 3 [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	0		0
Financial instruments, liabilities	0		0
Netting And Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	(162,776,000)		(99,618,000)
Hedged portion of gas stored underground	0		0
Available-for-sale Securities	0		0
Total assets	(162,776,000)		(99,618,000)
Financial instruments, liabilities	(186,451,000)		(124,447,000)
Netting And Collateral [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Netting And Collateral [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Netting And Collateral [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0		0
Netting And Collateral [Member] | Natural Gas Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	0		0
Financial instruments, liabilities	0		0
Netting And Collateral [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	(162,776,000)		(99,618,000)
Financial instruments, liabilities	$ (186,451,000)		$ (124,447,000)