SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)		9 Months Ended	12 Months Ended
	Jun. 01, 2010	Sep. 30, 2014 USD ($)	Sep. 30, 2016 USD ($) operating_segment change	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Related Party Transaction [Line Items]
Number of operating segments | operating_segment			2
Utility Plant, Depreciation and Amortization [Abstract]
Capital expenditures			$ 293,300,000	$ 289,800,000	$ 171,000,000
Amount by which the replacement cost exceeds (less than) the cost basis value of stored gas inventories			$ 11,400,000	20,400,000
Number of months of the original maturity of debt instruments purchased			3 months
Finance receivable, net			$ 11,800,000	11,200,000
Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Other Deferred Charges
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs			(13,200,000)	(13,000,000)
Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Long-Term Debt
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs			$ 13,200,000	$ 13,000,000
Laclede Gas
Utility Plant, Depreciation and Amortization [Abstract]
Average annual depreciation and amortization expense percentage		3.00%	3.00%	3.00%	3.00%
Capital expenditures			$ 197,800,000	$ 198,600,000	$ 163,000,000
Accruals for capital expenditures			$ 14,800,000	9,600,000	3,000,000
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
Accrued unbilled revenues			26,100,000	27,600,000
Laclede Gas | Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Other Deferred Charges
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs			(4,200,000)	(4,800,000)
Laclede Gas | Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Long-Term Debt
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs			4,200,000	4,800,000
Alagasco
Utility Plant, Depreciation and Amortization [Abstract]
Capital expenditures		$ 46,200,000	93,400,000	85,800,000	$ 46,200,000
Accruals for capital expenditures		$ 5,000,000	$ 6,800,000	3,100,000
Composite depreciation rate	3.10%		3.10%
Current refundable negative salvage costs to customers			$ 9,300,000
Noncurrent refundable negative salvage costs to customers			$ 18,700,000
Noncurrent refundable negative salvage costs period			3 years
Accrued unbilled revenues			$ 5,900,000	6,400,000
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	400,000
Alagasco | Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Other Deferred Charges
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs			(2,600,000)	(2,400,000)
Alagasco | Retrospective Effect is ASU had been Adopted | ASU 2015-03 | New Accounting Pronouncement, Early Adoption, Effect | Long-Term Debt
Utility Plant, Depreciation and Amortization [Abstract]
Deferred finance costs			2,600,000	2,400,000
Alagasco | Financing Receivables, Equal to Greater than 90 Days Past Due
Utility Plant, Depreciation and Amortization [Abstract]
Finance receivable 90 days past due			$ 400,000	$ 400,000