Fair Value Measurements (Details) (USD $)	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2014	Sep. 30, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets		$ 58,865,000	$ 121,320,000
Hedged portion of gas stored underground		40,492,000	44,758,000
Available-for-sale Securities		79,613,000	72,682,000
Total assets		178,970,000	238,760,000
Financial instruments, liabilities		21,856,000	1,543,000
Fair Value [Abstract]
Derivative, Cash Collateral		25,800,000	24,800,000
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		3,100,000	14,700,000
Derivative, Collateral, Right to Reclaim Cash		22,700,000	10,100,000
Debt Instrument Carrying Amount		2,460,000,000	2,460,000,000
Debt Instrument Fair Value		2,769,541,000
Other Matters [Abstract]
Asset Impairment Charges Park City	5,300,000
Nonrecurring fair value measurement	500,000
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		2,185,000	4,428,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		44,014,000	40,094,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		33,414,000	28,160,000
Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets		36,140,000	111,191,000
Financial instruments, liabilities		21,856,000	1,543,000
Fair Value [Abstract]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		0	0
Derivative, Collateral, Right to Reclaim Cash		0	0
Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets		22,725,000	10,129,000
Financial instruments, liabilities		0	0
Fair Value [Abstract]
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		3,033,000	14,705,000
Derivative, Collateral, Right to Reclaim Cash		22,725,000	10,124,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets		25,000	745,000
Hedged portion of gas stored underground		40,492,000	44,758,000
Available-for-sale Securities		44,014,000	40,094,000
Total assets		84,531,000	85,597,000
Financial instruments, liabilities		12,000	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		44,014,000	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] | Regulated 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		25,000	745,000
Financial instruments, liabilities		12,000	158,000
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets		105,138,000	226,326,000
Hedged portion of gas stored underground		0	0
Available-for-sale Securities		35,599,000	32,588,000
Total assets		140,737,000	258,914,000
Financial instruments, liabilities		93,900,000	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		2,185,000	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		33,414,000	28,160,000
Fair Value Inputs Level 2 [Member] | Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial instruments, assets		36,140,000	111,191,000
Financial instruments, liabilities		21,856,000	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		68,998,000	115,135,000
Financial instruments, liabilities		72,044,000	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] | Regulated 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		(46,298,000)	(105,751,000)
Hedged portion of gas stored underground		0	0
Available-for-sale Securities		0	0
Total assets		(46,298,000)	(105,751,000)
Financial instruments, liabilities		(72,056,000)	(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] | Regulated 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		(46,298,000)	(105,751,000)
Financial instruments, liabilities		$ (72,056,000)	$ (130,580,000)