COMMITMENTS AND CONTINGENCIES - Commitments (Details) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2018	Apr. 27, 2018
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,117.9
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	465.1
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 288.5
Spire STL Pipeline LLC | Subsequent Event
Long-term Purchase Commitment [Line Items]
Contractual commitment		$ 95.0