Financial Instruments (Narrative) (Details)		12 Months Ended
		Sep. 30, 2015 USD ($) counterparty MMcf bbl	Sep. 30, 2014 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash Surrender Value of Life Insurance		$ 45,300,000	$ 44,400,000
Net hedging gains (losses) in accumulated other comprehensive income (loss)		272,200,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		157,200,000
Pre-tax Net hedging gains (losses) reclassified within twelve months		168,100,000
After tax Net hedging gains (losses) reclassified within twelve months		$ 97,100,000
Number of counterparties in which the company holds over-the-counter swap positions | counterparty		16
Number of counterparties in net gain position | counterparty		15
Collateral Received from Counterparties by the Company		$ 29,700,000
Fair market value of derivative asset with a credit-risk related contingency		174,600,000
Hedging collateral deposits	[1]	11,124,000	2,734,000
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		34,900,000	35,300,000
Gross unrealized gain		6,500,000	8,400,000
Fixed Income Funds [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		8,000,000
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		4,300,000	6,600,000
Gross unrealized gain		2,600,000	$ 4,500,000
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		58,000,000
Over-The-Counter Swap Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Credit risk exposure per counterparty		18,000,000
Maximum credit risk exposure per counterparty		50,500,000
Hedging collateral deposits		$ 0
Over-The-Counter Swap Position [Member] | Credit Risk Related Contingency Feature [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of counterparties with a common credit-risk related contingency | counterparty		13
Exchange-Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 11,100,000
Fixed Price Sales Commitments MMCf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas | MMcf		17,900
Cash Flow Hedges Short Position [Member] | Natural Gas MMCf [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas | MMcf		180,500
Cash Flow Hedges Short Position [Member] | Crude Oil Bbls [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivative, crude oil | bbl		2,196,000
Cash Flow Hedges Long Position [Member] | Natural Gas MMCf [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas | MMcf		2,800

[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.