COMMITMENTS AND CONTINGENCIES - Commitments (Details) $ in Millions	9 Months Ended
	Jun. 30, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 1,311.6
Laclede Gas
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	518.7
Alagasco
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	$ 296.5