Financial Instruments (Narrative) (Details) $ in Thousands		9 Months Ended
		Jun. 30, 2015 USD ($) counterparty bbl MMcf	Sep. 30, 2014 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance		$ 45,200	$ 44,400
Net hedging gains/losses in accumulated other comprehensive income (loss)		242,400
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		139,900
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		141,200
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		$ 81,500
Number of counterparties in which the company holds over-the-counter swap positions | counterparty		15
Number of counterparties in net gain position | counterparty		14
Collateral Received from Counterparties by the Company		$ 32,400
Hedging collateral deposits	[1]	11,101	2,734
Fair market value of derivative asset with a credit-risk related contingency		151,900
Over-The-Counter Swap Position
Derivative Instruments, Gain (Loss) [Line Items]
Credit risk exposure per counterparty		17,200
Maximum credit risk exposure per counterparty		46,400
Hedging collateral deposits		$ 0
Fixed Price Purchase Commitments MMCf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas | MMcf		100
Fair Value Hedges MMCf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas | MMcf		16,100
Fixed Price Sales Commitments MMCf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas | MMcf		16,000
Natural Gas MMCf [Member] | Cash Flow Hedges Short Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas | MMcf		168,200
Natural Gas MMCf [Member] | Cash Flow Hedges Long Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas | MMcf		2,500
Crude Oil Bbls [Member] | Cash Flow Hedges Short Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, crude oil | bbl		2,178,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 11,100
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		36,900	35,300
Gross unrealized gain		8,500	8,400
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		6,400	6,600
Gross unrealized gain		4,300	$ 4,500
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability	[1]	554
Derivative, Notional Amount		$ 29,000
Credit Risk Related Contingency Feature [Member] | Over-The-Counter Swap Position
Derivative Instruments, Gain (Loss) [Line Items]
Number of counterparties with a common credit-risk related contingency | counterparty		12

[1] Netting Adjustments represent the impact of legally-enforceable master netting arrangements that allow the Company to net gain and loss positions held with the same counterparties. The net asset or net liability for each counterparty is recorded as an asset or liability on the Company’s balance sheet.