INCOME TAXES - Effective Income Tax Rate Variation from Stated Tax Rate (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
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		3.00%	1.80%		3.50%
Certain expenses capitalized on books and deducted on tax return		(3.70%)	(4.90%)		(9.70%)
Taxes related to prior years		(0.60%)	(0.70%)		(1.60%)
Other items – net		(2.50%)	(3.60%)		(2.20%)
Effective income tax rate		31.20%	27.60%		25.00%
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%	1.80%		3.30%
Certain expenses capitalized on books and deducted on tax return		(4.90%)	(4.50%)		(10.80%)
Taxes related to prior years		(0.80%)	(0.70%)		(1.60%)
Other items – net		(3.00%)	(3.30%)		(2.80%)
Effective income tax rate		29.10%	28.30%		23.10%
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%		37.70%