Financial Instruments (Narrative) (Details) (USD $)	6 Months Ended
	Mar. 31, 2012	Sep. 30, 2011
Cash surrender value of life insurance	$ 55,700,000	$ 54,800,000
Number of counterparties in which the company holds over-the-counter swap positions	11
Number of counterparties in net gain position	9
Fair market value of derivative asset with a credit-risk related contingency	82,000,000
Fair market value of derivative liability with a credit-risk related contingency	60,200,000
Hedging collateral deposits	18,872,000	19,701,000
Exploration And Production [Member]
Net hedging losses in accumulated other comprehensive income (loss)	59,000,000
After tax net hedging gains in accumulated other comprehensive income (loss)	34,400,000
Pre-tax Net hedging gains reclassified within twelve months	42,200,000
After tax Net hedging gains reclassified within twelve months	24,600,000
Energy Marketing [Member]
Net hedging losses in accumulated other comprehensive income (loss)	100,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	19,500,000
Equity Mutual Fund [Member]
Fair value	24,200,000	19,900,000
Gross unrealized gain	2,200,000
Gross unrealized loss		700,000
Insurance Company Stock [Member]
Fair value	5,400,000	4,500,000
Gross unrealized gain	2,900,000	2,100,000
Over-The-Counter Swap Position
Credit risk exposure per counterparty	13,300,000
Maximum credit risk exposure per counterparty	22,700,000
Over-The-Counter Swap Position | Credit Risk Related Contingency Feature [Member]
Number of counterparties with a common credit-risk related contingency	9
Over-The-Counter Crude Oil Swap Agreements [Member]
Hedging collateral deposits	8,800,000	14,200,000
Fixed Price Purchase Commitments cf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	1,800,000,000
Fixed Price Commitments Related To Withdrawal Of Storage Gas Cf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	300,000,000
Exchange-Traded Futures Contracts [Member]
Hedging collateral deposits	$ 10,100,000	$ 5,500,000
Fair Value Hedges Cf [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	9,500,000,000
Fixed Price Sales Commitments Cf Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	7,400,000,000
Natural Gas Cf [Member] | Exploration And Production [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	72,500,000,000
Natural Gas Cf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	8,800,000,000
Natural Gas Cf [Member] | Energy Marketing [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	5,300,000,000
Natural Gas Cf [Member] | Energy Marketing [Member] | Cash Flow Hedges Long Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	3,500,000,000
Natural Gas Cf [Member] | Pipeline And Storage [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	1,200,000,000
Crude Oil Bbls [Member] | Exploration And Production [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	2,502,000