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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended
	Mar. 31, 2015 Operating_Segment	Sep. 30, 2014	Jan. 06, 2015	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2013	Dec. 31, 2014
Related Party Transaction [Line Items]
Number of operating segments	2us-gaap_NumberOfOperatingSegments
Goodwill	$ 946.0us-gaap_Goodwill	$ 937.8us-gaap_Goodwill		$ 946.0us-gaap_Goodwill
Stockholders' Equity	1,611.6us-gaap_StockholdersEquity	1,508.4us-gaap_StockholdersEquity		1,611.6us-gaap_StockholdersEquity	1,106.6us-gaap_StockholdersEquity	1,046.3us-gaap_StockholdersEquity
Laclede Gas
Related Party Transaction [Line Items]
Accrued unbilled revenues	63.5	29.4		63.5
Goodwill	210.2	210.2		210.2
Accruals for capital expenditures	4.2	3.0
Stockholders' Equity	1,058.6	1,007.8		1,058.6	1,027.6	973.9
Alagasco
Related Party Transaction [Line Items]
Accrued unbilled revenues	15.0	5.2		15.0
Accruals for capital expenditures	4.7	5.0
Stockholders' Equity	908.9	849.6		908.9	416.8	364.7
Missouri Gas Energy (MGE)
Related Party Transaction [Line Items]
Goodwill	210.2	210.2		210.2
Missouri Gas Energy (MGE) \| Laclede Gas
Related Party Transaction [Line Items]
Goodwill	210.2	210.2		210.2
Alagasco
Related Party Transaction [Line Items]
Goodwill	735.8	727.6		735.8			727.6
Final reconciliation of net assets payments to acquire businesses			8.6	8.6
Alagasco \| Laclede Gas
Related Party Transaction [Line Items]
Goodwill	0	0		0
Common Stock Issued
Related Party Transaction [Line Items]
Stockholders' Equity	43.3	43.2		43.3	32.8	32.7
Common Stock Issued \| Laclede Gas
Related Party Transaction [Line Items]
Stockholders' Equity	0.1	0.1		0.1	0.1	0.1
Common Stock Issued \| Alagasco
Related Party Transaction [Line Items]
Stockholders' Equity	0	0		0	0	0
Common Stock Issued \| Restatement Adjustment \| Alagasco
Related Party Transaction [Line Items]
Stockholders' Equity	31.7			31.7
Paid-in Capital
Related Party Transaction [Line Items]
Stockholders' Equity	1,033.3	1,029.4		1,033.3	597.4	594.3
Paid-in Capital \| Laclede Gas
Related Party Transaction [Line Items]
Stockholders' Equity	746.2	744.0		746.2	741.0	738.1
Paid-in Capital \| Alagasco
Related Party Transaction [Line Items]
Stockholders' Equity	506.3	503.9		506.3	34.5	34.5
Paid-in Capital \| Restatement Adjustment \| Alagasco
Related Party Transaction [Line Items]
Stockholders' Equity	$ 31.7			$ 31.7