Financial Instruments (Narrative) (Details) $ in Thousands		6 Months Ended
		Mar. 31, 2016 USD ($) counterparty bbl MMcf	Sep. 30, 2015 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance		$ 38,900	$ 45,300
Net hedging gains/losses in accumulated other comprehensive income (loss)		252,900
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		146,700
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		180,300
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		104,600
Collateral Received from Counterparties by the Company		24,200
Hedging collateral deposits	[1]	9,963	11,124
Fair market value of derivative asset with a credit-risk related contingency		162,900
Over-The-Counter Swap Position
Derivative Instruments, Gain (Loss) [Line Items]
Maximum credit risk exposure per counterparty		$ 43,200
Over the Counter Swaps and Foreign Currency Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of counterparties in which the company holds over-the-counter swap positions | counterparty		16
Number of counterparties in net gain position | counterparty		15
Credit risk exposure per counterparty		$ 16,800
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 | MMcf		100
Withdrawal of Storage Mmcf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas | MMcf		500
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 | MMcf		13,800
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		13,200
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 | MMcf		139,500
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 | MMcf		3,100
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 | bbl		1,467,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 10,000
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		36,300	34,900
Gross unrealized gain		6,200	6,500
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		25,100	8,000
Gross unrealized loss		100
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		3,900	4,300
Gross unrealized gain		2,200	2,600
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability	[1]	0	$ 0
Derivative, Notional Amount		$ 84,500
Credit Risk Related Contingency Feature [Member] | Over the Counter Swaps and Foreign Currency Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of counterparties with a common credit-risk related contingency | counterparty		12

[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.