SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Laclede Gas	Sep. 30, 2013 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)	Sep. 30, 2014 Missouri Gas Energy (MGE) Laclede Gas	Sep. 30, 2013 Missouri Gas Energy (MGE) Laclede Gas	Aug. 31, 2014 Alagasco	Sep. 30, 2013 Alagasco	Sep. 01, 2013 Alagasco	Sep. 30, 2014 Alagasco Laclede Gas	Sep. 30, 2013 Alagasco Laclede Gas
Goodwill [Line Items]
Goodwill	$ 937.8	$ 247.1	$ 210.2	$ 247.1	$ 210.2	$ 210.2	$ 247.1	$ 210.2	$ 210.2	$ 247.1	$ 727.6	$ 0	$ 727.6	$ 0	$ 0