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ACQUISITION OF MGE Part 1 - Narrative (Details) (USD $)
12 Months Ended 0 Months Ended 0 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2013
Other Operating Expense [Member]
Sep. 30, 2013
Interest on Long-term Debt [Member]
Aug. 13, 2013
Laclede Gas [Member]
Sep. 01, 2013
Missouri Gas Energy (MGE) [Member]
Sep. 01, 2013
Missouri Gas Energy (MGE) [Member]
Gas Utility [Member]
May 29, 2013
Missouri Gas Energy (MGE) [Member]
Common Stock Issued
Aug. 13, 2013
Missouri Gas Energy (MGE) [Member]
Laclede Gas [Member]
Business Acquisition [Line Items]                    
Acquisition purchase price $ 975,000,000 $ 0 $ 0       $ 975,000,000      
Stock issued to finance acquisition                 10,000,000  
First mortgage bonds issued to finance acquisition 550,000,000 0 0     450,000,000       450,000,000
Goodwill 247,078,000 0         247,078,000 247,100,000    
Merger-related transaction costs $ 17,000,000 $ 200,000   $ 15,100,000 $ 1,900,000