Financial Instruments (Narrative) (Details)		12 Months Ended
		Sep. 30, 2017 USD ($) counterparty MMcf bbl	Sep. 30, 2016 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash Surrender Value of Life Insurance		$ 39,400,000	$ 39,700,000
Net hedging gains (losses) in accumulated other comprehensive income (loss)		35,500,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		20,800,000
Pre-tax Net hedging gains (losses) reclassified within twelve months		18,000,000
After tax Net hedging gains (losses) reclassified within twelve months		10,600,000
Fair market value of derivative asset with a credit-risk related contingency		26,000,000
Hedging collateral deposits	[1]	1,741,000	1,484,000
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		37,000,000	36,700,000
Gross unrealized gain		9,900,000	7,900,000
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		45,700,000	31,400,000
Gross unrealized loss		100,000
Gross unrealized gain			100,000
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		3,200,000	2,900,000
Gross unrealized gain		2,200,000	$ 1,600,000
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 89,200,000
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		17,500
Exchange-Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 1,700,000
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		16,400
Fixed Price Commitments Related To 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		1,100
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		16
Credit risk exposure per counterparty		$ 2,200,000
Maximum credit risk exposure per counterparty		6,000,000
Collateral Received from Counterparties by the Company		0
Hedging collateral deposits		$ 0
Over the Counter Swaps and Foreign Currency Forward Contracts [Member] | Credit Risk Related Contingency Feature [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of counterparties with a common credit-risk related contingency | counterparty		14
Cash Flow Hedges Short Position [Member] | Natural Gas MMCf [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas | MMcf		114,300
Cash Flow Hedges Short Position [Member] | Crude Oil Bbls [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivative, crude oil | bbl		3,459,000
Cash Flow Hedges Long Position [Member] | Natural Gas MMCf [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Nonmonetary notional amount of price risk cash flow hedge derivatives, natural gas | MMcf		1,000

[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.