PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) - USD ($)	3 Months Ended				9 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Extent of lump-sum payments recognized, percentage					100.00%
Medical insurance available until age is reached after early retirement					65 years
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Gain on lump-sum settlements	$ (200,000)			$ (12,500,000)	$ (2,400,000)	$ (12,500,000)
Special termination benefits	0			0	1,600,000	0
Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer					20,200,000
Anticipated contributions to pension plans for qualified trust					5,800,000
Pension Plans | Alagasco
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlements	12,600,000
Gain on lump-sum settlements	(200,000)	$ (2,200,000)		0	(2,400,000)	0
Contributions made by employer					0
Nonqualified Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer					400,000
Anticipated contributions to pension plans for qualified trust					100,000
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Special termination benefits	$ 0		$ 2,600,000	$ 0	2,600,000	$ 0
Anticipated contributions to pension plans for qualified trust					3,500,000
Anticipated payment directly to participants in postretirement plan					300,000
Postretirement Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer					$ 10,800,000