INTERIM FINANCIAL INFORMATION (UNAUDITED) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 222.3	$ 241.8	$ 694.5	$ 468.6	$ 147.1	$ 165.3	$ 397.6	$ 307.0	$ 1,627.2	$ 1,017.0	$ 1,125.5
Operating Income (Loss)	(8.4)	24.7	87.2	62.9	(9.7)	12.3	51.8	42.1	166.4	96.5	110.6
Net Income (Loss)	(14.9)	11.7	52.2	35.6	(9.6)	6.6	30.2	25.6	84.6	52.8	62.6
Basic Earnings (Loss) Per Share of Common Stock (in dollars per share)	$ (0.35)	$ 0.34	$ 1.59	$ 1.09	$ (0.30)	$ 0.25	$ 1.34	$ 1.14	$ 2.36	$ 2.03	$ 2.80
Diluted Earnings (Loss) Per Share of Common Stock (in dollars per share)	$ (0.35)	$ 0.33	$ 1.59	$ 1.09	$ (0.30)	$ 0.25	$ 1.34	$ 1.14	$ 2.35	$ 2.02	$ 2.79
Laclede Gas
Quarterly Financial Information [Line Items]
Total Operating Revenues	160.0	214.2	638.7	435.3	111.6	131.8	364.1	251.9	1,448.2	859.4	767.6
Operating Income (Loss)	7.2	22.5	74.4	62.3	(12.5)	7.2	50.7	42.1	166.4	87.5	90.8
Net Income (Loss)	(1.4)	12.0	44.2	35.3	(10.5)	3.8	29.8	25.7	90.1	48.8	49.9
Alagasco and MGE
Quarterly Financial Information [Line Items]
Net Income (Loss)	(11.3)
Missouri Gas Energy (MGE)
Quarterly Financial Information [Line Items]
Net Income (Loss)					(5.5)
Missouri Gas Energy (MGE) | Laclede Gas
Quarterly Financial Information [Line Items]
Net Income (Loss)					$ (5.5)