FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4.4	$ 7.4
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.4	7.4
Short-term debt	553.6	477.3
Long-term debt, including current portion	2,075.6	2,095.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.4	7.4
Short-term debt	553.6	477.3
Long-term debt, including current portion	2,074.0	2,210.3
Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.0	2.5	$ 2.1	$ 1.7
Spire Missouri | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.0	2.5
Short-term debt	345.3	203.0
Long-term debt, including current portion	874.4	973.9
Spire Missouri | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.0	2.5
Short-term debt	345.3	203.0
Long-term debt, including current portion	906.6	1,056.9
Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.1	$ 0.0	$ 7.2
Spire Alabama | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1
Short-term debt	142.5	169.9
Long-term debt, including current portion	322.6	247.8
Spire Alabama | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1
Short-term debt	142.5	169.9
Long-term debt, including current portion	321.7	269.4
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.4	7.4
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.0	2.5
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	553.6	477.3
Long-term debt, including current portion	2,074.0	2,210.3
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	345.3	203.0
Long-term debt, including current portion	906.6	1,056.9
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.0
Short-term debt	142.5	169.9
Long-term debt, including current portion	$ 321.7	$ 269.4