REGULATORY MATTERS - Narrative (Details) - USD ($)														3 Months Ended	12 Months Ended
	Sep. 30, 2017	Jul. 31, 2017	Apr. 30, 2017	Apr. 11, 2017	Apr. 04, 2017	Feb. 03, 2017	Jan. 31, 2017	Jan. 18, 2017	Nov. 30, 2016	Nov. 29, 2016	Sep. 30, 2016	Jul. 31, 2016	Jan. 01, 2014	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Mar. 31, 2018	Sep. 18, 2017	May 30, 2017	Apr. 15, 2016	Feb. 10, 2016	Dec. 01, 2015
Public Utilities, General Disclosures [Line Items]
Remaining Recovery Period of Certain Regulatory Assets for which No Return on Investment During Recovery Period is Provided															20 years
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided															15 years
Total Current Regulatory Liabilities	$ 22,600,000										$ 30,600,000			$ 22,600,000	$ 22,600,000	$ 30,600,000
Regulatory Liabilities	179,800,000										161,300,000			179,800,000	179,800,000	161,300,000
Spire Missouri
Public Utilities, General Disclosures [Line Items]
Regulatory preferred stock and debt securities issuance, amount requested				$ 25,500,000																	$ 550,000,000
Regulatory preferred stock and debt securities issuance, amount authorized	300,000,000													300,000,000	300,000,000					$ 300,000,000.0		$ 300,000,000.0
Approved return rate on equity				10.35%
Total Current Regulatory Liabilities	2,700,000										1,300,000			2,700,000	2,700,000	1,300,000
Regulatory Liabilities	83,900,000										68,600,000			83,900,000	83,900,000	68,600,000
Missouri Gas Energy (MGE)
Public Utilities, General Disclosures [Line Items]
Regulatory preferred stock and debt securities issuance, amount requested				$ 34,000,000
Approved return rate on equity				10.35%
Spire Alabama
Public Utilities, General Disclosures [Line Items]
Regulatory preferred stock and debt securities issuance, amount authorized	75,000,000.0													75,000,000.0	75,000,000.0
Total Current Regulatory Liabilities	12,000,000										22,700,000			12,000,000	12,000,000	22,700,000
Regulatory Liabilities	51,600,000										64,400,000			51,600,000	$ 51,600,000	64,400,000
Potential Performance-Based Adjustment, Percent															0.05%
Spire Alabama | Line of Credit
Public Utilities, General Disclosures [Line Items]
Line of credit facility borrowing capacity	200,000,000													200,000,000	$ 200,000,000
Line of Credit Facility, Maximum Amount Outstanding During Period														$ 25,000,000
APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved rate increase (decrease) amount	$ 2,700,000	$ 0	$ 0				$ 0		$ 2,500,000		2,700,000	$ 4,800,000			$ 10,700,000	7,800,000	$ 4,700,000
Rate increase (decrease), margin of index range	1.75%													1.75%	1.75%
Bond purchase agreement amount																			$ 75,000,000
Approved return rate on equity													10.80%
Maximum return on equity, percentage of prior year revenues																							4.00%
Infrastructure System Replacement Surcharge | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Approved rate increase, annualized revenue				$ 32,600,000
Regulatory preferred stock and debt securities issuance, amount requested				58,100,000
Infrastructure System Replacement Surcharge | Missouri Gas Energy (MGE)
Public Utilities, General Disclosures [Line Items]
Approved rate increase, annualized revenue				16,400,000
Regulatory preferred stock and debt securities issuance, amount requested				$ 50,400,000
Infrastructure System Replacement Surcharge | MoPSC | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Requested rate increase						$ 3,300,000					5,000,000
Approved rate increase, annualized revenue								$ 4,500,000
Approved rate increase (decrease) amount					$ 32,600,000			29,500,000
Infrastructure System Replacement Surcharge | MoPSC | Missouri Gas Energy (MGE)
Public Utilities, General Disclosures [Line Items]
Requested rate increase						$ 2,900,000					3,400,000
Recommended rate increase (decrease) amount					3,000,000					$ 3,400,000
Approved rate increase, annualized revenue								3,200,000
Approved rate increase (decrease) amount					16,400,000			$ 13,400,000
Updated filings | MoPSC | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Recommended rate increase (decrease) amount					$ 3,000,000					$ 4,500,000
Bond purchase agreement | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Bond purchase agreement amount																				$ 170,000,000
Minimum | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity													10.50%
Maximum | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity													10.95%
Amount subject to refund
Public Utilities, General Disclosures [Line Items]
Total Current Regulatory Liabilities	$ 8,200,000										9,300,000			$ 8,200,000	$ 8,200,000	9,300,000
Amount subject to refund | Spire Missouri
Public Utilities, General Disclosures [Line Items]
Total Current Regulatory Liabilities	0										0			0	0	0
Amount subject to refund | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Total Current Regulatory Liabilities	8,200,000										$ 9,300,000			8,200,000	8,200,000	$ 9,300,000
Amount subject to refund | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Total Current Regulatory Liabilities	6,300,000													6,300,000	6,300,000
Regulatory Liabilities	$ 12,300,000													$ 12,300,000	$ 12,300,000
Scenario, Forecast | Amount subject to refund | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Total Current Regulatory Liabilities																		$ 8,200,000