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Fair Value Measurements (Details) - USD ($) | Jun. 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)
Derivative Asset | 3,228,000 | 11,626,000
Hedged portion of gas stored underground |  | 52,578,000
Available-for-sale Securities | 89,611,000 | 72,701,000
Total assets | 92,839,000 | 136,905,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset | 0 | (82,865,000)
Derivative Liability | 109,372,000 | 240,819,000
Derivative, Cash Collateral |  | 50,400,000
Cash Held In Margin Accounts Classified Current Risk Management Asset | 0 | 6,775,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities | 0 | 43,575,000
Additional Fair Value Elements [Abstract] |  |
Debt Instrument Carrying Amount | 3,085,000,000 | 2,460,000,000
Debt Instrument Fair Value | 3,388,003,000 | 2,844,990,000
Equity Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 39,406,000 | 38,677,000
Fixed Income Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 15,892,000 |
Debt Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 31,429,000 | 31,394,000
Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 2,884,000 | 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 |  | 52,578,000
Available-for-sale Securities | 55,298,000 | 38,677,000
Total assets | 55,298,000 | 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 | 39,406,000 | 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 | 15,892,000 |
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 | 3,228,000 | 44,141,000
Hedged portion of gas stored underground |  | 0
Available-for-sale Securities | 34,313,000 | 34,024,000
Total assets | 37,541,000 | 78,165,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement | 109,372,000 | 323,684,000
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 | 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 | 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 | 31,429,000 | 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 | 2,884,000 | 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

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- Definition Fair value amount as of the balance sheet date of the hedged portion of natural gas stored underground in depleted gas reservoirs, aquifers, or salt caverns to meet seasonal and peak load demands, and also as insurance against unforeseen supply disruptions, and deemed to be a current asset because it is expected to be used within twelve months or in the normal operating cycle. + References No definition available. + Details
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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226003-175313 + Details
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