Financial Instruments (Narrative) (Details) (USD $)					9 Months Ended
	Jun. 30, 2013 entity		Sep. 30, 2012		Jun. 30, 2013 Exploration And Production [Member]	Jun. 30, 2013 Energy Marketing [Member]	Jun. 30, 2013 Equity Mutual Fund [Member]	Sep. 30, 2012 Equity Mutual Fund [Member]	Jun. 30, 2013 Insurance Company Stock [Member]	Sep. 30, 2012 Insurance Company Stock [Member]	Jun. 30, 2013 Over-The-Counter Swap Position	Jun. 30, 2013 Over-The-Counter Swap Position Credit Risk Related Contingency Feature [Member] entity	Jun. 30, 2013 Fixed Price Purchase Commitments MMCf [Member] Energy Marketing [Member] MMcf	Jun. 30, 2013 Exchange-Traded Futures Contracts [Member]	Sep. 30, 2012 Exchange-Traded Futures Contracts [Member]	Jun. 30, 2013 Fair Value Hedges MMCf [Member] MMcf	Jun. 30, 2013 Fixed Price Sales Commitments MMCf [Member] Energy Marketing [Member] MMcf	Jun. 30, 2013 Natural Gas MMCf [Member] Exploration And Production [Member] Cash Flow Hedges Short Position [Member] MMcf	Jun. 30, 2013 Natural Gas MMCf [Member] Energy Marketing [Member] MMcf	Jun. 30, 2013 Natural Gas MMCf [Member] Energy Marketing [Member] Cash Flow Hedges Short Position [Member] MMcf	Jun. 30, 2013 Natural Gas MMCf [Member] Energy Marketing [Member] Cash Flow Hedges Long Position [Member] MMcf	Jun. 30, 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	$ 57,000,000		$ 57,000,000
Fair value							30,100,000	24,800,000	6,300,000	4,800,000
Gross unrealized gain							4,100,000	2,600,000	3,900,000	2,300,000
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas																		208,000	7,100	5,600	1,500
Nonmonetary notional amount of price risk cash flow hedge derivatives, crude oil																						4,134,000
Net hedging gains/losses in accumulated other comprehensive income (loss)					61,600,000	1,700,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)					35,800,000	1,100,000
Pre-Tax Net Hedging Gain (Losses) Reclassified Within Twelve Months					40,100,000
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months					23,300,000
Number of counterparties in which the company holds over-the-counter swap positions	13
Number of counterparties with a common credit-risk related contingency												11
Number of counterparties in net gain position	11
Credit risk exposure per counterparty											5,800,000
Maximum credit risk exposure per counterparty											12,200,000
Fair market value of derivative asset with a credit-risk related contingency	48,800,000
Fair market value of derivative liability with a credit-risk related contingency	2,200,000
Hedging collateral deposits	$ 694,000	[1]	$ 364,000	[1]										$ 700,000	$ 400,000
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas													1,000			7,800	6,800

[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. In the tables above, presenting asset and liability information by gas and oil positions may result in negative assets or negative liabilities in Total column when a counterparty has issued both gas and oil swaps to the Company.