PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Narrative (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended								12 Months Ended				3 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Jan. 01, 2011	Aug. 01, 2007	Jan. 01, 2011 Laclede Gas	Aug. 01, 2007 Laclede Gas	Feb. 20, 2010 Missouri Gas Energy (MGE)	Sep. 30, 2014 Pension Plans		Sep. 30, 2013 Pension Plans	Sep. 30, 2012 Pension Plans	Sep. 30, 2014 Postretirement Plans	Sep. 30, 2014 Postretirement Plans		Sep. 30, 2013 Postretirement Plans	Sep. 30, 2012 Postretirement Plans
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 settlements	$ 22.1	$ 79.5							$ 22.1		$ 79.5	$ 60.1
Implementation period used to calculate return on plan assets (in years)	4 years
Gains or losses includible in cost amortized to extent exceeds benefit obligations or market-related value of plan assets, percentage.	10.00%
Recovery in rates for the qualified pension plans base allowance						15.5	4.8	10.0
Anticipated Contributions to pension plans for qualified trust	26.4
Anticipated contributions to pension plans for non-qualified plans	0.5
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
Net pension cost									27.4	[1]	17.5	20.1	9.1	9.1	[1]	9.5	9.5
Recovery in rates for postretirement benefit				9.5	7.6
Current investment policy targets an asset allocation to equity securities													60.00%	60.00%
Investment policy targets asset allocation to debt securities													40.00%	40.00%
Anticipated contributions to the postretirement plans for qualified trusts														18.1
Anticipated payment directly to participants in postretirement plan														0.3
Other Plans [Abstract]
401(K) plans, cost of defined contributions	$ 6.7	$ 5.0	$ 3.8

[1] Includes Alagasco.