Financial Instruments (Narrative) (Details) (USD $)	6 Months Ended
	Mar. 31, 2015 counterparty		Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance	$ 44,800,000		$ 44,400,000
Net hedging gains/losses in accumulated other comprehensive income (loss)	302,700,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	174,400,000
Pre-Tax Net Hedging Gain (Losses) Reclassified Within Twelve Months	167,400,000
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	96,500,000
Number of counterparties in which the company holds over-the-counter swap positions	15
Number of counterparties in net gain position	13
Collateral Received from Counterparties by the Company	50,900,000
Hedging collateral deposits	15,726,000	[1]	2,734,000	[1]
Fair market value of derivative asset with a credit-risk related contingency	189,500,000
Over-The-Counter Swap Position
Derivative Instruments, Gain (Loss) [Line Items]
Credit risk exposure per counterparty	23,200,000
Maximum credit risk exposure per counterparty	57,100,000
Hedging collateral deposits	0
Fixed Price Purchase Commitments MMCf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas	100
Fair Value Hedges MMCf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas	15,000
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	14,900
Natural Gas MMCf [Member] | Cash Flow Hedges Short Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas	157,400
Natural Gas MMCf [Member] | Cash Flow Hedges Long Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas	4,100
Crude Oil Bbls [Member] | Cash Flow Hedges Short Position [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, crude oil	2,568,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits	15,700,000.0
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value	37,100,000		35,300,000
Gross unrealized gain	9,000,000		8,400,000
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value	5,900,000		6,600,000
Gross unrealized gain	$ 3,800,000		$ 4,500,000
Credit Risk Related Contingency Feature [Member] | Over-The-Counter Swap Position
Derivative Instruments, Gain (Loss) [Line Items]
Number of counterparties with a common credit-risk related contingency	11

[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.