INCOME TAXES - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Tax Contingency [Line Items]
Federal income tax statutory rate		35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits		2.80%	3.00%	1.80%
Certain expenses capitalized on books and deducted on tax return		(3.40%)	(3.70%)	(4.90%)
Taxes related to prior years		(0.20%)	(0.60%)	(0.70%)
Other items – net		(1.70%)	(2.50%)	(3.60%)
Effective income tax rate		32.50%	31.20%	27.60%
Laclede Gas
Income Tax Contingency [Line Items]
Federal income tax statutory rate		35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits		2.80%	2.80%	1.80%
Certain expenses capitalized on books and deducted on tax return		(4.80%)	(4.90%)	(4.50%)
Taxes related to prior years		(0.20%)	(0.80%)	(0.70%)
Other items – net		(2.80%)	(3.00%)	(3.30%)
Effective income tax rate		30.00%	29.10%	28.30%
Alagasco
Income Tax Contingency [Line Items]
Federal income tax statutory rate	35.00%	35.00%	35.00%
State and local income taxes, net of federal income tax benefits	2.80%	2.80%	2.80%
Other items – net	(0.20%)	0.10%	0.10%
Effective income tax rate	37.60%	37.90%	37.90%