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, 2016	Sep. 30, 2015	Sep. 30, 2014
Alagasco | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average rate of future compensation increase	2.92%	3.00%	2.92%
Expected long-term rate of return on plan assets		7.50%
Alagasco | Pension Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.00%	4.25%	4.15%
Expected long-term rate of return on plan assets	7.00%		7.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate		3.45%	4.25%
Alagasco | Pension Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	4.05%	4.30%	4.25%
Expected long-term rate of return on plan assets	7.25%		7.25%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate		3.50%	4.30%
Alagasco | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.50%	4.40%	4.25%
Weighted average rate of future compensation increase				2.92%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate		3.60%	4.50%
Alagasco | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	4.75%	4.50%	4.75%
Alagasco | Postretirement Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	7.25%	7.25%	7.50%
Laclede Gas | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.40%	4.30%	4.70%
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		3.50%	4.40%
Laclede Gas | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.00%	4.15%	4.60%
Weighted average rate of future compensation increase		3.00%	3.00%	3.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate		3.15%	4.00%
Weighted average rate of future compensation increase		3.00%	3.00%
Laclede Gas | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets		6.00%	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%	7.75%
Missouri Gas Energy (MGE) | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.50%	4.45%	5.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate		3.50%	4.50%
Missouri Gas Energy (MGE) | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.30%	4.40%	4.95%
Expected long-term rate of return on plan assets		4.75%	5.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate		3.45%	4.30%
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, Alabama Gas Company, And Missouri Gas Energy | Pension Plans
Assumptions used to calculate benefit obligations [Abstract]
Weighted average rate of future compensation increase		3.00%	3.00%