COMMITMENTS AND CONTINGENCIES - Commitments (Details) $ in Millions	3 Months Ended
	Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,121.2
Spire Missouri
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	483.8
Spire Alabama
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 263.4