Financial Instruments (Narrative) (Details) $ in Thousands		6 Months Ended
		Mar. 31, 2018 USD ($) counterparty bbl MMcf	Sep. 30, 2017 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Net hedging gains (losses) in accumulated other comprehensive income (loss)		$ 8,100
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		3,800
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		(4,100)
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		(1,600)
Pre-Tax Net Hedging Gains (Losses) Reclassified After Twelve Months		12,200
After Tax Net Hedging Gains (Losses) Reclassified After Twelve Months		5,400
Fair market value of derivative asset with a credit-risk related contingency		16,800
Fair market value of derivative liability with a credit-risk related contingency		8,500
Hedging collateral deposits	[1]	$ 3,657	$ 1,741
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		8
Credit risk exposure per counterparty		$ 2,300
Maximum credit risk exposure per counterparty		8,600
Collateral Received by the Company		0
Hedging collateral deposits		$ 0
Withdrawal of Storage Gas Mmcf [Member] | Energy Marketing [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk fair value hedge derivatives, natural gas | MMcf		600
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		21,700
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		21,100
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		106,600
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		4,284,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 3,700
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gross unrealized gain		9,400	9,900
Sale proceeds		1,500
Gross realized gain		400
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gross unrealized loss		500	100
Sale proceeds		1,500
Gross realized loss		100
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gross unrealized gain		1,900	$ 2,200
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 91,100
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		14

[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.