Financial Instruments (Narrative) (Details) $ in Thousands		9 Months Ended
		Jun. 30, 2016 USD ($) counterparty bbl MMcf	Sep. 30, 2015 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance		$ 39,300	$ 45,300
Net hedging gains/losses in accumulated other comprehensive income (loss)		124,200
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		72,000
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		93,200
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		54,000
Collateral Received from Counterparties by the Company		0
Hedging collateral deposits	[1]	3,008	11,124
Fair market value of derivative asset with a credit-risk related contingency		$ 81,600
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		17
Number of counterparties in net gain position | counterparty		14
Credit risk exposure per counterparty		$ 9,000
Maximum credit risk exposure per counterparty		25,800
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		13,300
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		12,700
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		178,000
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		1,700
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		1,722,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 3,000
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		37,000	34,900
Gross unrealized gain		6,600	6,500
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		31,300	8,000
Gross unrealized gain		100
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		3,800	4,300
Gross unrealized gain		2,100	2,600
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability	[1]	0	$ 0
Derivative, Notional Amount		$ 81,500
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		13

[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.