Fair Value Measurements (Details) - USD ($) $ in Thousands		Sep. 30, 2015		Sep. 30, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		$ (82,509)	[1]	$ (46,298)	[2]
Derivative Asset		9,600		58,865
Hedged portion of gas stored underground		43,901		40,492
Available-for-sale Securities		74,200		79,613
Total assets		127,701		178,970
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		(125,983)	[1]	(72,056)	[2]
Derivative Liability		120,107		21,856
Additional Fair Value Elements [Abstract]
Cash Held in Margin Accounts		43,500		25,800
Cash Held In Margin Accounts Used to Offset Risk Management Liabilities		34,600		3,100
Cash Held In Margin Accounts Classified Current Risk Management Asset		8,900		22,700
Debt Instrument Carrying Amount		2,460,000		2,460,000
Debt Instrument Fair Value		2,669,323
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,072		2,185
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		40,619		44,014
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		32,509		33,414
Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		0	[1]	0	[2]
Derivative Asset		746		36,140
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		0	[1]	0	[2]
Derivative Liability		120,107		21,856
Additional Fair Value Elements [Abstract]
Cash Held In Margin Accounts Used to Offset Risk Management Liabilities		0		0
Cash Held In Margin Accounts Classified Current Risk Management Asset		0		0
Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		(82,509)	[1]	(46,298)	[2]
Derivative Asset		8,854		22,725
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		(125,983)	[1]	(72,056)	[2]
Derivative Liability		0		0
Additional Fair Value Elements [Abstract]
Cash Held In Margin Accounts Used to Offset Risk Management Liabilities		34,620		3,033
Cash Held In Margin Accounts Classified Current Risk Management Asset		8,854		22,725
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0		25
Hedged portion of gas stored underground		43,901		40,492
Available-for-sale Securities		40,619		44,014
Total assets		84,520		84,531
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		0		12
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		40,619		44,014
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]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0		0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		0		0
Fair Value Inputs Level 1 [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0		25
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		0		12
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	[3]	92,109		105,138
Hedged portion of gas stored underground	[3]	0		0
Available-for-sale Securities	[3]	33,581		35,599
Total assets	[3]	125,690		140,737
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[3]	246,090		93,900
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]	1,072		2,185
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	[3]	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	[3]	32,509		33,414
Fair Value Inputs Level 2 [Member] | Regulated Distribution Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	[3]	746		36,140
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[3]	120,107		21,856
Fair Value Inputs Level 2 [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	[3]	91,363		68,998
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[3]	125,983		72,044
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0		0
Hedged portion of gas stored underground		0		0
Available-for-sale Securities		0		0
Total assets		0		0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		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]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0		0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		0		0
Fair Value Inputs Level 3 [Member] | Nonregulated Segment [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0		0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		$ 0		$ 0

[1] This column reflects adjustments to our gross financial instrument assets and liabilities to reflect netting permitted under our master netting agreements and the relevant authoritative accounting literature. In addition, as of September 30, 2015 we had $43.5 million of cash held in margin accounts to collateralize certain financial instruments. Of this amount, $34.6 million was used to offset current and noncurrent risk management liabilities under master netting agreements and the remaining $8.9 million is classified as current risk management assets.

[2] This column reflects adjustments to our gross financial instrument assets and liabilities to reflect netting permitted under our master netting agreements and the relevant authoritative accounting literature. In addition, as of September 30, 2014 we had $25.8 million of cash held in margin accounts to collateralize certain financial instruments. Of this amount, $3.1 million was used to offset current and noncurrent risk management liabilities under master netting agreements and the remaining $22.7 million is classified as current risk management assets.

[3] Our Level 2 measurements consist of over-the-counter options and swaps, which are valued using a market-based approach in which observable market prices are adjusted for criteria specific to each instrument, such as the strike price, notional amount or basis differences, municipal and corporate bonds, which are valued based on the most recent available quoted market prices and money market funds which are valued at cost.