ACQUISITIONS - Missouri Gas Energy (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended										0 Months Ended			12 Months Ended			0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2014 Laclede Gas	Sep. 30, 2013 Laclede Gas	Sep. 30, 2012 Laclede Gas	Sep. 01, 2013 Laclede Gas	Sep. 30, 2014 Missouri Gas Energy (MGE)	Sep. 30, 2013 Missouri Gas Energy (MGE)	Sep. 01, 2013 Missouri Gas Energy (MGE)	Feb. 14, 2014 Missouri Gas Energy (MGE) Laclede Gas	Dec. 20, 2013 Missouri Gas Energy (MGE) Laclede Gas	Sep. 01, 2013 Missouri Gas Energy (MGE) Laclede Gas	Sep. 30, 2014 Missouri Gas Energy (MGE) Laclede Gas	Sep. 30, 2013 Missouri Gas Energy (MGE) Laclede Gas	Sep. 01, 2013 Scenario, Previously Reported Missouri Gas Energy (MGE)	Sep. 01, 2013 Scenario, Previously Reported Missouri Gas Energy (MGE) Laclede Gas
Business Acquisition [Line Items]
Acquisition purchase price	$ (23.9)	$ 975.0	$ 0	$ (23.9)	$ 975.0	$ 0									$ 475.0		$ 975.0
Fair value adjustment of assets											23.9
Proceeds from final reconciliation of acquisition of Missouri Gas Energy												11.0
Total consideration transferred													940.2	940.2
Goodwill	$ 937.8	$ 247.1		$ 210.2	$ 247.1		$ 210.2	$ 210.2	$ 247.1	$ 210.2				$ 210.2	$ 247.1	$ 247.1	$ 247.1