PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Targeted and Actual Plan Assets by Category (Details)	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Alagasco | Pension Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%	100.00%
Actual allocation of plan assets	100.00%	100.00%
Alagasco | Pension Plans | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%	46.00%
Actual allocation of plan assets	52.90%	46.00%
Alagasco | Pension Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	29.00%	33.00%
Actual allocation of plan assets	27.90%	29.00%
Alagasco | Pension Plans | Other
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	11.00%	21.00%
Actual allocation of plan assets	19.20%	25.00%
Alagasco | Postretirement Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%
Actual allocation of plan assets	100.00%	100.00%
Alagasco | Postretirement Plans | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%
Actual allocation of plan assets	59.70%	60.00%
Alagasco | Postretirement Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	40.00%
Actual allocation of plan assets	40.30%	40.00%
Laclede Gas | Pension Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%	100.00%
Actual allocation of plan assets	100.00%	100.00%
Laclede Gas | Pension Plans | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	52.00%	50.00%
Actual allocation of plan assets	48.40%	51.20%
Laclede Gas | Pension Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	48.00%	50.00%
Actual allocation of plan assets	50.10%	48.70%
Laclede Gas | Pension Plans | Other
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	0.00%	0.00%
Actual allocation of plan assets	1.50%	0.10%
Laclede Gas | Postretirement Plans
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	100.00%
Actual allocation of plan assets	100.00%	100.00%
Laclede Gas | Postretirement Plans | Equity markets
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	60.00%
Actual allocation of plan assets	59.60%	59.00%
Laclede Gas | Postretirement Plans | Debt securities
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	40.00%
Actual allocation of plan assets	39.70%	39.00%
Laclede Gas | Postretirement Plans | Other
Targeted and actual plan assets by category [Abstract]
Targeted allocation of plan assets	0.00%
Actual allocation of plan assets	0.70%	2.00%