SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - New Accounting Pronouncements (Details) - Accounting Standards Update 2015-03 - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Spire | Deferred charges
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs	$ 14.8	$ 13.0	$ 12.3
Spire | Long-term debt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs	14.9	13.0	11.9
Laclede Gas | Deferred charges
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs	4.1	4.2	4.5
Laclede Gas | Long-term debt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt issuance costs	4.0	4.2	4.4
Alagasco | 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