COMMITMENTS AND CONTINGENCIES - Commitments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Long-term Purchase Commitment [Line Items]
Natural and propane gas purchased	$ 731.7	$ 433.4	$ 397.3
Capital Addition Purchase Commitments
Long-term Purchase Commitment [Line Items]
Estimated capital expenditures for fiscal 2015	318.3
Gas Utility | Capital Addition Purchase Commitments
Long-term Purchase Commitment [Line Items]
Estimated capital expenditures for fiscal 2015	290.6
Gas Marketing | Capital Addition Purchase Commitments
Long-term Purchase Commitment [Line Items]
Estimated capital expenditures for fiscal 2015	27.7
Laclede Gas
Long-term Purchase Commitment [Line Items]
Natural and propane gas purchased	816.9	469.1	414.8
Laclede Gas | Gas Utility | Capital Addition Purchase Commitments
Long-term Purchase Commitment [Line Items]
Estimated capital expenditures for fiscal 2015	205.6
Laclede Pipeline Company
Long-term Purchase Commitment [Line Items]
Ownership interest in subsidiaries (in percent)	100.00%
Alagasco
Long-term Purchase Commitment [Line Items]
Natural and propane gas purchased	33.8	50.0	51.0
Alagasco | Inventories
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	134.0
Minimum volume purchase commitment required (in Mmcf)	111,000
Missouri Gas Energy (MGE) | Gas Utility | Capital Addition Purchase Commitments
Long-term Purchase Commitment [Line Items]
Estimated capital expenditures for fiscal 2015	85.0
Subsidiary of Common Parent | Laclede Gas
Long-term Purchase Commitment [Line Items]
Minimum total payments required for natural gas contracts	1,345.1
Purchase Commitment | Subsidiary of Common Parent | Laclede Gas
Long-term Purchase Commitment [Line Items]
Annual purchase commitment	$ 1.0
Commitment termination minimum required notification period	6 months