ACQUISITIONS - Pro Forma Information, Revenues and Earnings (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Business Acquisition [Line Items]
Total Operating Revenues		$ 2,187.1	$ 2,051.5
Net Income		$ 133.5	$ 102.0
Basic earnings (loss) per share (in dollars per share)		$ 3.11	$ 2.50
Diluted earnings (loss) per share (in dollars per share)		$ 3.10	$ 2.49
Laclede Gas
Business Acquisition [Line Items]
Total Operating Revenues			$ 1,518.2
Net Income			$ 83.6
Basic earnings (loss) per share (in dollars per share)
Diluted earnings (loss) per share (in dollars per share)
Missouri Gas Energy (MGE)
Business Acquisition [Line Items]
Pro forma, Total Operating Revenues	$ 556.8	$ 554.2	$ 22.0
Pro forma, Net Income (Loss)	$ 39.9	$ 39.5	$ 1.8
Pro forma, Earnings (Loss) Per Share	$ 0.92	$ 1.10	$ 0.07
Alagasco
Business Acquisition [Line Items]
Pro forma, Total Operating Revenues	$ 479.2	$ 19.7	$ 0.0
Pro forma, Net Income (Loss)	$ 48.0	$ (2.9)	$ 0.0
Pro forma, Earnings (Loss) Per Share	$ 1.11	$ (0.08)	$ 0.00