Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 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	$ (93,400)	$ (46,298)
Derivative Asset	12,695	58,865
Hedged portion of gas stored underground	65,717	40,492
Available-for-sale Securities	79,489	79,613
Total assets	157,901	178,970
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(124,723)	(72,056)
Derivative Liability	52,140	21,856
Additional Fair Value Elements [Abstract]
Derivative, Cash Collateral	31,300	25,800
Cash Held In Margin Accounts Classified Current Risk Management Asset	10,800	22,700
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	20,500	3,100
Debt Instrument Carrying Amount	2,460,000	2,460,000
Debt Instrument Fair Value	2,659,908	2,769,541
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,217	2,185
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	44,854	44,014
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	33,418	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	0
Derivative Asset	1,889	36,140
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	0	0
Derivative Liability	52,140	21,856
Additional Fair Value Elements [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]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(93,400)	(46,298)
Derivative Asset	10,806	22,725
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(124,723)	(72,056)
Derivative Liability	0	0
Additional Fair Value Elements [Abstract]
Cash Held In Margin Accounts Classified Current Risk Management Asset	10,806	22,725
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	20,517	3,033
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	65,717	40,492
Available-for-sale Securities	44,854	44,014
Total assets	110,571	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	44,854	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	106,095	105,138
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	34,635	35,599
Total assets	140,730	140,737
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	176,863	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	1,217	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	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	33,418	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	1,889	36,140
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	52,140	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	104,206	68,998
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	124,723	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