INTERIM FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 279.3	$ 249.3	$ 609.3	$ 399.4	$ 204.2	$ 275.2	$ 877.4	$ 619.6		$ 1,537.3	$ 1,976.4	$ 1,627.2
Operating Income (Loss)	(7.7)	35.3	167.7	87.0	(8.5)	36.0	157.7	87.3		282.3	272.5	166.4
Net Income (Loss)	$ (14.2)	$ 10.7	$ 100.8	$ 46.9	$ (18.7)	$ 14.1	$ 94.4	$ 47.1		$ 144.2	$ 136.9	$ 84.6
Basic Earnings Per Share of Common Stock (in dollars per share)	$ (0.31)	$ 0.24	$ 2.32	$ 1.08	$ (0.43)	$ 0.32	$ 2.18	$ 1.09		$ 3.26	$ 3.16	$ 2.36
Diluted Earnings Per Share of Common Stock (in dollars per share)	$ (0.31)	$ 0.24	$ 2.31	$ 1.08	$ (0.43)	$ 0.32	$ 2.18	$ 1.09		$ 3.24	$ 3.16	$ 2.35
Laclede Gas
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 144.3	$ 179.3	$ 446.7	$ 317.2	$ 151.0	$ 187.5	$ 615.7	$ 462.4		$ 1,087.5	$ 1,416.6	$ 1,448.2
Operating Income (Loss)	5.4	29.4	87.0	65.1	5.5	34.5	80.6	64.8		186.9	185.4	166.4
Net Income (Loss)	(1.7)	13.9	54.3	39.4	(3.6)	20.0	49.9	39.0		105.9	105.3	$ 90.1
Alagasco
Quarterly Financial Information [Line Items]
Total Operating Revenues	46.2	74.0	166.0	82.3	52.2	73.7	233.3	120.0	$ 417.2	368.5	479.2
Operating Income (Loss)	(17.1)	9.3	80.4	18.9	(12.5)	3.9	77.4	20.4	62.2	91.5	89.2
Net Income (Loss)	$ (8.8)	$ 4.0	$ 48.1	$ 9.9	$ (9.6)	$ 0.7	$ 46.3	$ 10.6	$ 33.0	$ 53.2	$ 48.0