COMMITMENTS AND CONTINGENCIES - Commitments (Details) - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2018	Apr. 27, 2018
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,240.5
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	510.7
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 328.0
Spire STL Pipeline LLC
Long-term Purchase Commitment [Line Items]
Contractual commitment		$ 100.0