Fair Value Measurements (Details) (USD $)	Dec. 31, 2013	Sep. 30, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	$ 146,427,000	$ 121,320,000
Hedged portion of gas stored underground	76,151,000	44,758,000
Available-for-sale Securities	75,390,000	72,682,000
Total assets	297,968,000	238,760,000
Financial instruments, liabilities	36,000	1,543,000
Fair Value [Abstract]
Derivative, Cash Collateral	16,700,000	24,800,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	9,000,000	10,100,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	7,700,000	14,700,000
Debt Instrument Carrying Amount	2,460,000,000	2,460,000,000
Debt Instrument Fair Value	2,661,390,000	2,676,487,000
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,376,000	4,428,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	44,000,000	40,094,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	28,014,000	28,160,000
Natural Gas Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	134,812,000	111,191,000
Financial instruments, liabilities	36,000	1,543,000
Fair Value [Abstract]
Cash Held In Margin Accounts Classified Current Risk Management Asset	0	0
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	0	0
Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	11,615,000	10,129,000
Financial instruments, liabilities	0	0
Fair Value [Abstract]
Cash Held In Margin Accounts Classified Current Risk Management Asset	9,001,000	10,124,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	7,707,000	14,705,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	184,000	745,000
Hedged portion of gas stored underground	76,151,000	44,758,000
Available-for-sale Securities	44,000,000	40,094,000
Total assets	120,335,000	85,597,000
Financial instruments, liabilities	1,172,000	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	44,000,000	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	184,000	745,000
Financial instruments, liabilities	1,172,000	158,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets	238,677,000	226,326,000
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	31,390,000	32,588,000
Total assets	270,067,000	258,914,000
Financial instruments, liabilities	108,006,000	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	3,376,000	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	28,014,000	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	134,812,000	111,191,000
Financial instruments, liabilities	36,000	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	103,865,000	115,135,000
Financial instruments, liabilities	107,970,000	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	(92,434,000)	(105,751,000)
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	0	0
Total assets	(92,434,000)	(105,751,000)
Financial instruments, liabilities	(109,142,000)	(130,580,000)
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	(92,434,000)	(105,751,000)
Financial instruments, liabilities	$ (109,142,000)	$ (130,580,000)