Financial Instruments (Narrative) (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2016 USD ($) counterparty bbl MMcf	Sep. 30, 2016 USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Cash surrender value of life insurance	$ 39,800	$ 39,700
Net hedging gains/losses in accumulated other comprehensive income (loss)	28,600
After tax net hedging gains (losses) in accumulated other comprehensive income (loss)	16,600
Pre-Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	23,900
After Tax Net Hedging Gains (Losses) Reclassified Within Twelve Months	13,800
Fair market value of derivative asset with a credit-risk related contingency	24,400
Fair market value of derivative liability with a credit-risk related contingency	6,600
Hedging collateral deposits	0	1,484	[1]
Energy Marketing [Member] | Economic Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Gain	$ 200
Nonmonetary notional amount of price risk derivative instrument natural gas | MMcf	300
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	10
Credit risk exposure per counterparty	$ 3,700
Maximum credit risk exposure per counterparty	10,300
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	400
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	12,600
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	12,100
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	156,300
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	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	2,334,000
Exchange Traded Futures Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Collateral Received by the Company	$ 2,200
Hedging collateral deposits	0
Equity Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value	33,000	36,700
Gross unrealized gain	5,700	7,900
Fixed Income Mutual Fund [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value	38,300	31,400
Gross unrealized gain		100
Gross unrealized loss	100
Insurance Company Stock [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value	3,700	2,900
Gross unrealized gain	2,400	$ 1,600
Foreign Currency Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount	$ 75,500
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.