Financial Instruments (Narrative) (Details) (USD $)	12 Months Ended
	Sep. 30, 2012 entity	Sep. 30, 2011
Cash surrender value of life insurance	$ 57,000,000	$ 54,800,000
Number of counterparties in which the company holds over-the-counter swap positions	12
Number of counterparties with a common credit-risk related contingency	10
Fair market value of derivative asset with a credit-risk related contingency	14,000,000
Fair market value of derivative liability with a credit-risk related contingency	23,900,000
Hedging collateral deposits	364,000	19,701,000
Exploration And Production [Member]
Net hedging gains (losses) in accumulated other comprehensive income (loss)	900,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	500,000
Pre-tax Net hedging gains (losses) reclassified within twelve months	21,900,000
After tax Net hedging gains (losses) reclassified within twelve months	12,700,000
Energy Marketing [Member]
Net hedging gains (losses) in accumulated other comprehensive income (loss)	(2,800,000)
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	(1,700,000)
Pipeline And Storage [Member]
Net hedging gains (losses) in accumulated other comprehensive income (loss)	(700,000)
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	(400,000)
Equity Mutual Fund [Member]
Fair value	24,800,000	19,900,000
Gross unrealized gain	2,600,000
Gross unrealized loss		700,000
Insurance Company Stock [Member]
Fair value	4,800,000	4,500,000
Gross unrealized gain	2,300,000	2,100,000
Over-The-Counter Swap Position [Member]
Credit risk exposure per counterparty	6,400,000
Maximum credit risk exposure per counterparty	11,000,000
Over-The-Counter Crude Oil Swap Agreements [Member]
Hedging collateral deposits		14,200,000
Fixed Price Purchase Commitments Bcf [Member] | Energy Marketing [Member] | Fair Value Hedges Short Position [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	1,100,000,000
Exchange-Traded Futures Contracts [Member]
Hedging collateral deposits	$ 400,000	$ 5,500,000
Fair Value Hedges Bcf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	10,200,000,000
Fixed Price Sales Commitments Bcf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	8,700,000,000
Natural Gas Bcf [Member] | Exploration And Production [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	133,500,000,000
Natural Gas Bcf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	5,700,000,000
Crude Oil Bbls [Member] | Exploration And Production [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivative	2,316,000
Fixed Price Commitments Related To Withdrawal Of Storage Gas Cf [Member] | Energy Marketing [Member] | Fair Value Hedges Short Position [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	400,000,000