PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Assumptions Used to Calculate Net Periodic Cost and Benefit Obligations (Details)	12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2012
Pension Plans
Assumptions used to calculate benefit obligations [Abstract]
Weighted average rate of future compensation increase (all plans)	3.00%		3.00%
Postretirement Plans
Assumptions used to calculate benefit obligations [Abstract]
Weighted average rate of future compensation increase (all plans)	3.00%		3.00%
Alagasco | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average rate of future compensation increase	2.92%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate			0.00%
Alagasco | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.25%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.40%		0.00%
Laclede Gas | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.70%		3.95%		5.10%
Weighted average rate of future compensation increase	3.00%	[1]	3.00%	[1]	3.00%	[1]
Expected long-term rate of return on plan assets	7.75%	[1]	7.75%	[1]	7.75%	[1]
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.30%		4.70%
Laclede Gas | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.60%		3.80%		5.05%
Weighted average rate of future compensation increase	3.00%	[1]	3.00%	[1]	3.00%	[1]
Expected long-term rate of return on plan assets			7.75%		7.75%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.15%		4.60%
Missouri Gas Energy (MGE) | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	5.00%		5.05%		0.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.45%		5.00%
Missouri Gas Energy (MGE) | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.95%		5.00%		0.00%
Expected long-term rate of return on plan assets			5.75%		0.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.40%		4.95%
Minimum | Alagasco | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.00%
Weighted average rate of future compensation increase	7.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.15%
Minimum | Alagasco | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	4.75%
Minimum | Laclede Gas | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	6.25%
Minimum | Missouri Gas Energy (MGE) | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	3.75%
Maximum | Alagasco | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.05%
Weighted average rate of future compensation increase	7.25%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.25%
Maximum | Alagasco | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	7.50%
Maximum | Laclede Gas | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	7.75%
Maximum | Missouri Gas Energy (MGE) | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	5.75%

[1] Assumptions for weighted average rate of future compensation increase and expected long-term rate of return on plan assets are the same for both Laclede Gas and MGE plans.