SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) (USD $)	12 Months Ended							0 Months Ended	1 Months Ended	12 Months Ended						0 Months Ended	12 Months Ended		0 Months Ended
	Sep. 30, 2014 change Operating_Segment	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	Jun. 01, 2010 Alagasco	Sep. 30, 2014 Alagasco	Sep. 30, 2014 Alagasco	Sep. 30, 2013 Alagasco	Jun. 01, 2010 Alagasco	Sep. 30, 2014 Missouri Gas Energy (MGE)	Sep. 30, 2013 Missouri Gas Energy (MGE)	Sep. 01, 2013 Missouri Gas Energy (MGE)	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. 02, 2014 Alagasco	Aug. 31, 2014 Alagasco	Sep. 30, 2013 Alagasco	Sep. 01, 2013 Alagasco	Sep. 30, 2014 Alagasco Laclede Gas	Sep. 30, 2013 Alagasco Laclede Gas	Sep. 01, 2013 Scenario, Previously Reported Missouri Gas Energy (MGE)	Sep. 01, 2013 Scenario, Previously Reported Missouri Gas Energy (MGE) Laclede Gas	Sep. 30, 2014 Gas Utility	Sep. 30, 2013 Gas Utility
Related Party Transaction [Line Items]
Number of operating segments	3
Utility Plant, Depreciation and Amortization [Abstract]
Average annual depreciation and amortization expense percentage				3.00%	3.20%	3.10%
Capital expenditures	$ 171,000,000	$ 130,800,000	$ 108,800,000	$ 163,000,000	$ 128,500,000	$ 106,700,000			$ 7,700,000
Accruals for capital expenditures				3,000,000	4,700,000	9,700,000				5,000,000
Composite depreciation rate (percent)								3.10%
Current refundable negative salvage costs to customers												13,400,000
Noncurrent refundable negative salvage costs to customers												26,800,000
Noncurrent refundable negative salvage costs period (in years)								5 years
The amount by which the replacement cost exceeds (less than) the cost basis value of stored gas inventories	11,400,000	13,300,000
Goodwill	937,800,000	247,100,000		210,200,000	247,100,000		210,200,000						210,200,000	247,100,000	210,200,000		210,200,000	247,100,000		727,600,000	0	727,600,000	0	0	247,100,000	247,100,000
Total consideration transferred																940,200,000	940,200,000		1,600,000,000
Accrued unbilled revenues																											29,400,000	26,700,000
Gas imbalances									0	0
Number of PGA discretionary changes	3
Months between PGA discretionary changes	2 months
Cost savings retention percentage	10.00%
Maximum amount of cost savings allowable	3,000,000.0
Number of months of the original maturity of debt instruments purchased	3 months
Gross Receipts taxes recorded in regulated gas distribution operating revenues	96,900,000	40,800,000	35,900,000	76,300,000						20,600,000
Finance receivable, net	10,900,000	10,800,000
Threshold period past due for collection										90 days
Threshold period past due for write-off of financing receivable										12 months
Finance receivable 90 days past due									300,000	300,000	400,000
Write off on an allowance for credit losses										$ 300,000	$ 400,000