Financial Instruments (Narrative) (Details) $ in Thousands		9 Months Ended
		Jun. 30, 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)		$ (25,600)
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		(20,600)
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		(21,900)
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		(14,900)
Fair market value of derivative asset with a credit-risk related contingency		10,600
Fair market value of derivative liability with a credit-risk related contingency		31,900
Hedging collateral deposits	[1]	$ 2,283	$ 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		3
Credit risk exposure per counterparty		$ 3,500
Maximum credit risk exposure per counterparty		6,700
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		24,800
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		24,200
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		131,600
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		1,300
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		4,314,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 2,300
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gross unrealized gain		9,600	9,900
Sale proceeds		1,500
Gross realized gain		400
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gross unrealized loss		600	100
Sale proceeds		1,500
Gross realized loss		100
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gross unrealized gain		1,800	$ 2,200
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 90,800
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.