PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Assumptions Used to Calculate Net Periodic Cost and Benefit Obligations (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Postretirement Plans
Assumptions used to calculate benefit obligations [Abstract]
Weighted average rate of future compensation increase	3.00%	3.00%	3.00%
Alagasco | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate				3.63%
Weighted average rate of future compensation increase	2.92%	2.92%		3.71%
Expected long-term rate of return on plan assets				7.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average rate of future compensation increase	2.92%	3.00%	2.92%
Alagasco | Pension Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.00%	4.15%
Weighted average rate of future compensation increase	7.00%	7.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.15%	4.25%	4.15%
Alagasco | Pension Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.05%	4.25%
Weighted average rate of future compensation increase	7.25%	7.25%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.25%	4.30%	4.25%
Alagasco | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.40%	4.25%		4.26%
Expected long-term rate of return on plan assets					7.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.40%	4.50%	4.40%
Alagasco | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets		4.75%	4.75%
Alagasco | Postretirement Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets		7.50%	7.25%
Laclede Gas | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.30%	4.70%		3.95%
Weighted average rate of future compensation increase		3.00%	3.00%		3.00%
Expected long-term rate of return on plan assets		7.75%	7.75%		7.75%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.30%	4.40%	4.30%
Laclede Gas | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.15%	4.60%		3.80%
Expected long-term rate of return on plan assets					7.75%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.15%	4.00%	4.15%
Laclede Gas | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets		6.25%	6.25%
Laclede Gas | Postretirement Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets		7.75%	7.75%
Missouri Gas Energy (MGE) | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.45%	5.00%		5.05%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.45%	4.50%	4.45%
Missouri Gas Energy (MGE) | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.40%	4.95%		5.00%
Expected long-term rate of return on plan assets		5.00%			5.75%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	4.40%	4.30%	4.40%
Missouri Gas Energy (MGE) | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets			3.75%
Missouri Gas Energy (MGE) | Postretirement Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets			5.75%
Laclede Gas and MGE | Pension Plans
Assumptions used to calculate benefit obligations [Abstract]
Weighted average rate of future compensation increase	3.00%	3.00%	3.00%
Laclede Gas and MGE | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average rate of future compensation increase		3.00%	3.00%		3.00%