REGULATORY MATTERS - Narrative (Details) - USD ($)													12 Months Ended
	Oct. 01, 2017	Dec. 01, 2016	Oct. 26, 2016	Sep. 30, 2016	Jul. 31, 2016	May 31, 2016	May 19, 2016	Apr. 30, 2016	Jan. 31, 2016	Dec. 01, 2015	Nov. 12, 2015	Jan. 01, 2014	Sep. 30, 2016	Sep. 30, 2015	Nov. 11, 2016	Apr. 15, 2016	Feb. 10, 2016	Jun. 30, 2015
Public Utilities, General Disclosures [Line Items]
Regulatory assets for which no return on investment during recovery period is provided				$ 404,800,000									$ 404,800,000	$ 372,400,000
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided													15 years
Laclede Gas
Public Utilities, General Disclosures [Line Items]
Regulatory assets for which no return on investment during recovery period is provided				404,800,000									$ 404,800,000	372,400,000
Regulatory preferred stock and debt securities issuance, amount authorized				300,000,000									$ 300,000,000				$ 300,000,000.0	$ 518,000,000.0
Regulatory preferred stock and debt securities issuance, amount requested																$ 550,000,000
Alagasco
Public Utilities, General Disclosures [Line Items]
Potential performance-based adjustment													0.05%
APSC | Alagasco
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount				$ (2,700,000)	$ (4,800,000)			$ (5,800,000)	$ 0				$ 7,800,000	$ 4,700,000
Approved return rate on equity												10.80%
Maximum return on equity, percentage of prior year revenues										4.00%
Rate increase (decrease), margin of index range				1.75%									1.75%
Infrastructure System Replacement Surcharge | MoPSC | Laclede Gas
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount						$ 25,000,000	$ 5,400,000				$ 4,400,000
Approved rate increase, annualized revenue										$ 19,600,000
Requested rate increase				$ 5,000,000
Infrastructure System Replacement Surcharge | MoPSC | Missouri Gas Energy (MGE)
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount						$ 10,300,000	$ 3,600,000				$ 1,900,000
Approved rate increase, annualized revenue										$ 6,700,000
Requested rate increase				$ 3,400,000
Subsequent Event | Laclede Gas
Public Utilities, General Disclosures [Line Items]
Regulatory preferred stock and debt securities issuance, authorized debt issuance - remaining available															$ 300,000,000
Subsequent Event | APSC | Alagasco
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount	$ (4,800,000)		$ (1,300,000)
Minimum | APSC | Alagasco
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity												10.50%
Maximum | APSC | Alagasco
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity												10.95%
Scenario, Forecast | APSC | Alagasco
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount		$ (2,700,000)