SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)			9 Months Ended	12 Months Ended
	Jan. 06, 2015 USD ($)	Jun. 01, 2010	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Operating_Segment reporting_unit change	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2015 USD ($)	Aug. 31, 2014 USD ($)	Dec. 31, 2012 USD ($)	Sep. 30, 2012 USD ($)
Related Party Transaction [Line Items]
Number of operating segments | Operating_Segment				2
Utility Plant, Depreciation and Amortization [Abstract]
Capital expenditures				$ 289,800,000	$ 171,000,000		$ 130,800,000
Amount by which the replacement cost exceeds (less than) the cost basis value of stored gas inventories				20,400,000	11,400,000
Goodwill			$ 937,800,000	$ 946,000,000	937,800,000
Number of reporting units | reporting_unit				1
Number of PGA discretionary changes | change				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
Finance receivable, net			10,900,000	$ 11,200,000	10,900,000
Common stock equity			1,508,400,000	1,573,600,000	1,508,400,000		1,046,300,000				$ 601,600,000
Common Stock Outstanding
Utility Plant, Depreciation and Amortization [Abstract]
Common stock equity			43,200,000	43,300,000	43,200,000		$ 32,700,000				22,600,000
Alagasco
Utility Plant, Depreciation and Amortization [Abstract]
Goodwill			$ 727,600,000	$ 735,800,000	$ 727,600,000				$ 735,800,000
Final reconciliation of net assets payments to acquire businesses	$ 8,200,000
Laclede Gas
Utility Plant, Depreciation and Amortization [Abstract]
Average annual depreciation and amortization expense percentage			3.00%	3.00%	3.00%		3.20%
Capital expenditures				$ 198,600,000	$ 163,000,000		$ 128,500,000
Accruals for capital expenditures				9,600,000	3,000,000		4,700,000
Goodwill			$ 210,200,000	$ 210,200,000	210,200,000
Number of reporting units | reporting_unit				1
Accrued unbilled revenues			29,400,000	$ 27,600,000	29,400,000
Common stock equity			1,007,800,000	1,037,800,000	1,007,800,000		973,900,000				491,300,000
Laclede Gas | Common Stock Outstanding
Utility Plant, Depreciation and Amortization [Abstract]
Common stock equity			100,000	100,000	100,000		100,000				$ 100,000
Alagasco
Utility Plant, Depreciation and Amortization [Abstract]
Capital expenditures			46,200,000	85,800,000	46,200,000	$ 86,000,000	$ 86,000,000
Accruals for capital expenditures				$ 3,100,000	5,000,000	5,500,000
Composite depreciation rate		3.10%		3.10%
Current refundable negative salvage costs to customers				$ 10,800,000
Noncurrent refundable negative salvage costs to customers				$ 27,000,000
Noncurrent refundable negative salvage costs period				4 years
Accrued unbilled revenues			5,200,000	$ 6,400,000	5,200,000
Threshold period past due for collection				90 days
Threshold period past due for write-off of financing receivable				12 months
Write off on an allowance for credit losses				$ 400,000	300,000
Common stock equity			849,600,000	874,600,000	849,600,000	384,600,000				$ 360,500,000
Alagasco | Common Stock Outstanding
Utility Plant, Depreciation and Amortization [Abstract]
Common stock equity			0	0	0	$ 0				$ 0
Alagasco | Common Stock Outstanding | Restatement Adjustment
Utility Plant, Depreciation and Amortization [Abstract]
Common stock equity								$ 31,700,000
Alagasco | Financing Receivables, Equal to Greater than 90 Days Past Due
Utility Plant, Depreciation and Amortization [Abstract]
Finance receivable 90 days past due			$ 300,000	400,000	$ 300,000
Alagasco | Affiliated Entity
Utility Plant, Depreciation and Amortization [Abstract]
Employee-related integration transactions costs				$ 2,800,000