PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) - USD ($)	3 Months Ended			6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2017	Mar. 31, 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				$ 11,800,000.0
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	$ (11,900,000)	$ 2,200,000		$ (11,900,000)	$ 2,200,000
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	1,900,000.0
Gain (Loss) on lump-sum settlements	(400,000)	2,200,000		(400,000)	2,200,000
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	36,300,000.0
Gain (Loss) on lump-sum settlements	(11,500,000)	0		(11,500,000)	0
Special termination benefits	0	0	$ 1,600,000	0	1,600,000
Contributions made by employer				12,300,000
Anticipated contributions to pension plans for qualified trust				16,700,000
Nonqualified Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer				100,000
Anticipated contributions to pension plans for qualified trust				500,000
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				4,100,000
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