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