Financial Instruments (Narrative) (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2019 USD ($) counterparty MMcf bbl		Sep. 30, 2018 USD ($)	[1]	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Foreign Currency Forward Contract Hedge Duration	7 years
Net hedging gains (losses) in accumulated other comprehensive income (loss)	$ 6,400
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	4,600
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	1,700
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	1,200
Fair market value of derivative asset with a credit-risk related contingency	7,600
Fair market value of derivative liability with a credit-risk related contingency	5,700
Hedging collateral deposits	$ 1,983	[1]	$ 3,441		$ 3,657	$ 1,741
Cash Flow and Fair Value Commodity Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedge Duration	5 years
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	18
Number of counterparties in net gain position | counterparty	10
Credit risk exposure per counterparty	$ 1,100
Maximum credit risk exposure per counterparty	4,000
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	200
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	23,500
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	23,300
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	97,500
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	4,500
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	3,414,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits	$ 2,000
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 84,600
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	15

[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.