Financial Instruments (Narrative) (Details)		12 Months Ended
		Sep. 30, 2016 USD ($) counterparty MMcf bbl	Sep. 30, 2015 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash Surrender Value of Life Insurance		$ 39,700,000	$ 45,300,000
Net hedging gains (losses) in accumulated other comprehensive income (loss)		111,800,000
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)		64,800,000
Pre-tax Net hedging gains (losses) reclassified within twelve months		83,200,000
After tax Net hedging gains (losses) reclassified within twelve months		48,200,000
Collateral Received from Counterparties by the Company		0
Fair market value of derivative asset with a credit-risk related contingency		73,000,000
Hedging collateral deposits	[1]	1,484,000	11,124,000
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		36,700,000	34,900,000
Gross unrealized gain		7,900,000	6,500,000
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		31,400,000	8,000,000
Gross unrealized gain		100,000
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value		2,900,000	4,300,000
Gross unrealized gain		1,600,000	$ 2,600,000
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount		$ 78,500,000
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		12,700
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
Exchange-Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Hedging collateral deposits		$ 1,500,000
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		12,100
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		500
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		$ 7,000,000
Maximum credit risk exposure per counterparty		20,600,000
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		13
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		159,200
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		1,755,000
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		700

[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.