PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) - USD ($)	3 Months Ended				9 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Extent of lump-sum payments recognized, percentage					100.00%
Lump-sum payments recognized as settlement					$ 12,600,000
Contributions made by employer					$ 8,800,000
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 (Loss) on lump-sum settlements	$ 0	$ 2,200,000	$ 200,000		$ 11,900,000	$ 2,400,000
Special termination benefits	0		0		0	1,600,000
Pension Plans | Alagasco
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlement	1,900,000
Gain (Loss) on lump-sum settlements	0	400,000	200,000		400,000	2,400,000
Contributions made by employer					0
Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlement	36,300,000
Gain (Loss) on lump-sum settlements	0	$ 11,500,000	0		11,500,000	0
Special termination benefits	0		0	$ 1,600,000	0	1,600,000
Contributions made by employer					22,500,000
Anticipated contributions to pension plans for qualified trust	6,500,000				6,500,000
Nonqualified Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer					200,000
Anticipated contributions to pension plans for qualified trust	400,000				400,000
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Special termination benefits	0		0	$ 2,600,000	0	2,600,000
Anticipated contributions to pension plans for qualified trust	5,100,000				5,100,000
Anticipated payment directly to participants in postretirement plan					400,000
Postretirement Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Special termination benefits	$ 0		$ 0		$ 0	$ 2,600,000