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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - New Accounting Pronouncements (Details) - Accounting Standards Update 2015-03 - USD ($) $ in Millions | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Spire | Other deferred charges |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Debt issuance costs | $ (12.5) | $ (13.0) | $ (12.6)
Spire | Long-term debt |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Debt issuance costs | 12.5 | 13.0 | 12.6
Laclede Gas | Other deferred charges |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Debt issuance costs | (4.1) | (4.2) | (4.6)
Laclede Gas | Long-term debt |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Debt issuance costs | 4.1 | 4.2 | 4.6
Alagasco | Other deferred charges |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Debt issuance costs | (2.3) | (2.4) | (2.4)
Alagasco | Long-term debt |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Debt issuance costs | $ 2.3 | $ 2.4 | $ 2.4

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