Fair Value Measurements (Details) - USD ($)		Sep. 30, 2017	Sep. 30, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		$ 0	$ (32,515,000)	[1],[2]
Derivative Asset		3,239,000	11,626,000	[1]
Hedged portion of gas stored underground			52,578,000
Available-for-sale Securities		88,409,000	72,701,000
Total assets		91,648,000	136,905,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		0	(82,865,000)	[1],[2]
Derivative Liability		112,398,000	240,819,000	[1]
Cash Held in Margin Accounts			50,400,000
Cash Held In Margin Accounts Used to Offset Risk Management Liabilities		0	43,575,000
Cash Held In Margin Accounts Classified Current Risk Management Asset		0	6,775,000
Additional Fair Value Elements [Abstract]
Debt Instrument Carrying Amount		3,085,000,000	2,460,000,000
Debt Instrument Fair Value		3,382,272,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		41,097,000	38,677,000
Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		16,371,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		29,104,000	31,394,000
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		1,837,000	2,630,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0	0
Hedged portion of gas stored underground	[1]		52,578,000
Available-for-sale Securities		57,468,000	38,677,000
Total assets		57,468,000	91,255,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		0	0
Fair Value Inputs Level 1 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		41,097,000	38,677,000
Fair Value Inputs Level 1 [Member] | Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		16,371,000
Fair Value Inputs Level 1 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Fair Value Inputs Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	[3]	3,239,000	44,141,000	[1]
Hedged portion of gas stored underground	[3]		0
Available-for-sale Securities	[3]	30,941,000	34,024,000
Total assets	[3]	34,180,000	78,165,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	[3]	112,398,000	323,684,000	[1]
Fair Value Inputs Level 2 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	[3]	0	0
Fair Value Inputs Level 2 [Member] | Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	[3]	0
Fair Value Inputs Level 2 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	[3]	29,104,000	31,394,000
Fair Value Inputs Level 2 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	[3]	1,837,000	2,630,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0	0
Hedged portion of gas stored underground			0
Available-for-sale Securities		0	0
Total assets		0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		0	0
Fair Value Inputs Level 3 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Fair Value Inputs Level 3 [Member] | Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		0
Fair Value Inputs Level 3 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		0	0
Fair Value Inputs Level 3 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities		$ 0	$ 0

[1] Our financial instruments and hedged portion of gas stored underground include assets and liabilities related to our natural gas marketing operations, which are classified as “held for sale” on our consolidated balance sheets at September 30, 2016.

[2] This column reflects adjustments to our gross financial instrument assets and liabilities to reflect netting permitted under our master netting agreements and the relevant authoritative accounting literature. As of September 30, 2016 we had $50.4 million of cash held in margin accounts to collateralize certain financial instruments. Of this amount, $43.6 million was used to offset current and noncurrent risk management liabilities under master netting agreements and the remaining $6.8 million is classified as current risk management assets.

[3] Our Level 2 measurements consist of over-the-counter options and swaps, which are valued using a market-based approach in which observable market prices are adjusted for criteria specific to each instrument, such as the strike price, notional amount or basis differences, municipal and corporate bonds, which are valued based on the most recent available quoted market prices and money market funds which are valued at cost.