REGULATORY MATTERS - Spire Alabama (Details) - USD ($) $ in Millions								12 Months Ended
	Oct. 01, 2018	Feb. 01, 2018	Nov. 30, 2017	Sep. 30, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 01, 2014	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 02, 2018
Public Utilities, General Disclosures [Line Items]
Regulatory costs								$ 30.6
Amounts returned to customers				$ 22.6				35.7	$ 22.6
Amount to be refunded				179.8				390.3	179.8
Spire Alabama
Public Utilities, General Disclosures [Line Items]
Regulatory costs		$ 12.8
Amounts returned to customers				12.0				7.6	12.0
Amount to be refunded				51.6				$ 38.9	51.6
Potential Performance-Based Adjustment, Percent								0.05%
APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity	10.40%						10.80%
Maximum return on equity, percentage of prior year revenues								4.00%
Approved rate increase (decrease) amount				2.7	$ 0.0	$ 0.0		$ 9.7	10.7	$ 7.8
Requested rate increase			$ 8.5
Rate increase (decrease), margin of index range								1.75%
Amount to be refunded								$ 5.2
Line of credit facility borrowing capacity								200.0
Principal amount outstanding								25.0
ESR | APSC
Public Utilities, General Disclosures [Line Items]
Extraordinary O&M expenses in excess of								1.0
Single force majeure								0.3
Multiple force majeures								0.4
Revenue variances								$ 0.4
ESR term								9 years
Underfunded ESR balances amortization term								5 years
ESR annual limitation								$ 0.7
Minimum | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity	10.15%						10.50%
Equity limitation as percent of capitalization							55.50%
Maximum | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Approved return rate on equity	10.65%						10.95%
Equity limitation as percent of capitalization							56.50%
Subsequent Event | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Regulatory preferred stock and debt securities issuance, amount authorized											$ 90.0
Amount subject to refund
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers				8.2				5.2	8.2
Amount subject to refund | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers				8.2				5.2	8.2
Amount subject to refund | APSC | Spire Alabama
Public Utilities, General Disclosures [Line Items]
Amounts returned to customers				$ 6.3				$ 7.2	$ 6.3	$ 8.3