Financial Instruments (Narrative) (Details) (USD $)			6 Months Ended
	Mar. 31, 2013 entity	Sep. 30, 2012	Mar. 31, 2013 Exploration And Production [Member]	Mar. 31, 2013 Energy Marketing [Member]	Mar. 31, 2013 Equity Mutual Fund [Member]	Sep. 30, 2012 Equity Mutual Fund [Member]	Mar. 31, 2013 Insurance Company Stock [Member]	Sep. 30, 2012 Insurance Company Stock [Member]	Mar. 31, 2013 Credit Risk Related Contingency Feature [Member]	Mar. 31, 2013 Over-The-Counter Swap Position [Member]	Mar. 31, 2013 Over-The-Counter Swap Position [Member] Credit Risk Related Contingency Feature [Member] entity	Mar. 31, 2013 Fixed Price Purchase Commitments MMCf [Member] Energy Marketing [Member] MMcf	Sep. 30, 2012 Exchange-Traded Futures Contracts [Member]	Mar. 31, 2013 Fair Value Hedges MMCf [Member] MMcf	Mar. 31, 2013 Fixed Price Sales Commitments MMCf [Member] Energy Marketing [Member] MMcf	Mar. 31, 2013 Natural Gas MMCf [Member] Exploration And Production [Member] Cash Flow Hedges Short Position [Member] MMcf	Mar. 31, 2013 Natural Gas MMCf [Member] Energy Marketing [Member] MMcf	Mar. 31, 2013 Natural Gas MMCf [Member] Energy Marketing [Member] Cash Flow Hedges Short Position [Member] MMcf	Mar. 31, 2013 Natural Gas MMCf [Member] Energy Marketing [Member] Cash Flow Hedges Long Position [Member] MMcf	Mar. 31, 2013 Crude Oil Bbls [Member] Exploration And Production [Member] Cash Flow Hedges Short Position [Member] bbl
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance	$ 56,500,000	$ 57,000,000
Fair value					30,100,000	24,800,000	5,800,000	4,800,000
Gross unrealized gain					4,200,000	2,600,000	3,400,000	2,300,000
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas																152,300	7,600	4,500	3,100
Nonmonetary notional amount of price risk cash flow hedge derivatives, crude oil																				3,390,000
Net hedging gains/losses in accumulated other comprehensive income (loss)			(36,500,000)	(200,000)
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)			(21,200,000)	(100,000)
Pre-Tax Net Hedging Gain (Losses) Reclassified Within Twelve Months			(12,200,000)	(300,000)
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months			(7,100,000)	(200,000)
Number of counterparties in which the company holds over-the-counter swap positions	11
Number of counterparties with a common credit-risk related contingency											9
Number of counterparties in net gain position	3
Credit risk exposure per counterparty										1,500,000
Maximum credit risk exposure per counterparty										2,400,000
Fair market value of derivative asset with a credit-risk related contingency	2,700,000
Fair market value of derivative liability with a credit-risk related contingency									38,700,000
Hedging collateral deposits	$ 750,000	$ 364,000								$ 800,000			$ 400,000
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas												1,400		6,800	5,400