Financial Instruments (Narrative) (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2018 USD ($) counterparty bbl MMcf		Sep. 30, 2018 USD ($)	[1]	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Net hedging gains (losses) in accumulated other comprehensive income (loss)	$ 27,600
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	17,900
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	15,200
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	9,900
Fair market value of derivative asset with a credit-risk related contingency	21,100
Fair market value of derivative liability with a credit-risk related contingency	900
Hedging collateral deposits	$ 2,784	[1]	$ 3,441		$ 4,465	$ 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	18
Number of counterparties in net gain position | counterparty	14
Credit risk exposure per counterparty	$ 2,400
Maximum credit risk exposure per counterparty	6,600
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	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	25,600
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	25,400
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	97,300
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	2,400
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	3,735,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits	$ 2,800
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 89,500
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	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.