SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Goodwill [Line Items]
Goodwill	$ 946.0		$ 937.8
Laclede Gas
Goodwill [Line Items]
Goodwill	210.2		210.2
Missouri Gas Energy (MGE)
Goodwill [Line Items]
Goodwill	210.2		210.2
Missouri Gas Energy (MGE) | Laclede Gas
Goodwill [Line Items]
Goodwill	210.2		210.2
Alagasco
Goodwill [Line Items]
Goodwill	735.8	727.6	727.6
Alagasco | Laclede Gas
Goodwill [Line Items]
Goodwill	$ 0		$ 0