Financial Instruments (Narrative) (Details) (USD $)	3 Months Ended
	Dec. 31, 2011	Sep. 30, 2011
Cash surrender value of life insurance	$ 54,900,000	$ 54,800,000
Number of counterparties in which the company holds over-the-counter swap positions	11
Number of counterparties in net gain position	10
Fair market value of derivative asset with a credit-risk related contingency	70,900,000
Fair market value of derivative liability with a credit-risk related contingency	44,700,000
Hedging collateral deposits	25,118,000	19,701,000
Exploration And Production [Member]
Net hedging gains in accumulated other comprehensive income (loss)	58,100,000
After tax net hedging gains in accumulated other comprehensive income (loss)	33,600,000
Pre-tax Net hedging gains reclassified within twelve months	36,800,000
After tax Net hedging gains reclassified within twelve months	21,200,000
Energy Marketing [Member]
Net hedging gains in accumulated other comprehensive income (loss)	2,500,000
After tax net hedging gains in accumulated other comprehensive income (loss)	1,500,000
Equity Mutual Fund [Member]
Fair value	21,700,000	19,900,000
Gross unrealized gain	300,000
Gross unrealized loss		700,000
Insurance Company Stock [Member]
Fair value	4,200,000	4,500,000
Gross unrealized gain	1,800,000	2,100,000
Over-The-Counter Swap Position [Member]
Credit risk exposure per counterparty	10,500,000
Maximum credit risk exposure per counterparty	19,200,000
Over-The-Counter Swap Position [Member] | Credit Risk Related Contingency Feature [Member]
Number of counterparties with a common credit-risk related contingency	8
Over-The-Counter Crude Oil Swap Agreements [Member]
Hedging collateral deposits	20,600,000	14,200,000
Commitments Related To The Withdrawal of Storage Gas [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	1,800,000,000
Exchange-Traded Futures Contracts [Member]
Hedging collateral deposits	4,500,000	5,500,000
Exchange-Traded Futures Contracts [Member] | Hedging Collateral Deposits [Member]
Hedging collateral deposits	$ 4,500,000
Fair Value Hedges Bcf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	10,600,000,000
Fixed Price Sales Commitments Bcf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk fair value hedge derivatives	8,800,000,000
Natural Gas cf [Member] | Exploration And Production [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	54,700,000,000
Natural Gas cf [Member] | Energy Marketing [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	8,300,000,000
Natural Gas cf [Member] | Energy Marketing [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	5,700,000,000
Natural Gas cf [Member] | Energy Marketing [Member] | Cash Flow Hedges Long Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	2,600,000,000
Crude Oil Bbls [Member] | Exploration And Production [Member] | Cash Flow Hedges Short Position [Member]
Nonmonetary notional amount of price risk cash flow hedge derivatives	2,331,000