Financial Instruments (Narrative) (Details) $ in Thousands		6 Months Ended
		Mar. 31, 2017 USD ($) counterparty bbl MMcf	Sep. 30, 2016 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance		$ 40,500	$ 39,700
Net hedging gains/losses in accumulated other comprehensive income (loss)		62,200
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		36,300
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		35,900
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months		21,000
Fair market value of derivative asset with a credit-risk related contingency		41,000
Fair market value of derivative liability with a credit-risk related contingency		600
Hedging collateral deposits	[1]	$ 1,771	1,484
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		16
Number of counterparties in net gain position | counterparty		13
Credit risk exposure per counterparty		$ 4,800
Maximum credit risk exposure per counterparty		10,000
Collateral Received by the Company		0
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 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		15,800
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		15,100
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		138,800
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		1,600
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		3,051,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 1,800
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		34,700	36,700
Gross unrealized gain		7,400	7,900
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		38,500	31,400
Gross unrealized gain			100
Gross unrealized loss		100
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		3,600	2,900
Gross unrealized gain		2,400	$ 1,600
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 72,500
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		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.