PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) (USD $)	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
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
Amortization period for transition obligation excluded from postretirement benefit cost (in years)			20 years
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial loss	$ 2,000,000	$ 1,700,000	$ 3,900,000	$ 3,500,000
Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer			9,100,000
Pension Plans | Missouri Gas Energy (MGE)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlements			12,700,000
Anticipated contributions to pension plans for qualified trust			17,300,000
Pension Plans | Alagasco
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Lump-sum payments recognized as settlements			2,600,000
Contributions made by employer			0
Anticipated contributions to pension plans for qualified trust			0
Amortization of actuarial loss	0	700,000	0	2,000,000
Nonqualified Pension Plans | Laclede Gas
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer			200,000
Nonqualified Pension Plans | Missouri Gas Energy (MGE)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Anticipated contributions to pension plans for qualified trust			300,000
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Anticipated contributions to pension plans for qualified trust			13,600,000
Amortization of actuarial loss	1,200,000	1,500,000	2,500,000	3,000,000
Anticipated payment directly to participants in postretirement plan			300,000
Postretirement Plans | Missouri Gas Energy (MGE)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions made by employer			4,400,000
Postretirement Plans | Alagasco
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amortization of actuarial loss	$ 0	$ (400,000)	$ 0	$ (500,000)