PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Assumptions Used to Calculate Net Periodic Cost and Benefit Obligations (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Spire Alabama | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average rate of future compensation increase	3.00%	3.00%	2.92%
Expected long-term rate of return on plan assets	7.25%	7.50%
Spire Alabama | Pension Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.45%	4.25%	4.15%
Expected long-term rate of return on plan assets			7.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	3.65%	3.45%
Spire Alabama | Pension Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.50%	4.30%	4.25%
Expected long-term rate of return on plan assets			7.25%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	3.70%	3.50%
Spire Alabama | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.60%	4.50%	4.40%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	3.80%	3.60%
Spire Alabama | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	4.00%	4.50%	4.75%
Spire Alabama | Postretirement Plans | Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	6.25%	7.25%	7.50%
Spire Missouri | Pension Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.50%	4.40%	4.30%
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	3.75%	3.50%
Spire Missouri | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.15%	4.00%	4.15%
Weighted average rate of future compensation increase	3.00%	3.00%	3.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	3.60%	3.15%
Weighted average rate of future compensation increase	3.00%	3.00%
Spire Missouri | Postretirement Plans | Minimum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	5.75%	6.00%	6.25%
Spire Missouri | 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	3.50%	4.50%	4.45%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	3.70%	3.50%
Missouri Gas Energy (MGE) | Postretirement Plans
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.45%	4.30%	4.40%
Expected long-term rate of return on plan assets	5.50%	4.75%	5.00%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	3.60%	3.45%
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%