FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 7.4	$ 5.2	$ 13.8	$ 16.1
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.4	5.2
Short-term debt	477.3	398.7
Long-term debt, including current portion	2,095.0	2,070.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.4	5.2
Short-term debt	477.3	398.7
Long-term debt, including current portion	2,210.3	2,257.1
Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.5	2.1	1.7	3.7
Spire Missouri | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.5	2.1
Short-term debt	203.0	243.7
Long-term debt, including current portion	973.9	804.1
Spire Missouri | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.5	2.1
Short-term debt	203.0	243.7
Long-term debt, including current portion	1,056.9	900.4
Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1	0.0	$ 7.2	$ 5.6
Spire Alabama | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1
Short-term debt	169.9	82.0
Long-term debt, including current portion	247.8	247.6
Spire Alabama | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1
Short-term debt	169.9	82.0
Long-term debt, including current portion	269.4	275.5
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.4	5.2
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	2.5	2.1
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	477.3	398.7
Long-term debt, including current portion	2,210.3	2,257.1
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	203.0	243.7
Long-term debt, including current portion	1,056.9	900.4
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	169.9	82.0
Long-term debt, including current portion	$ 269.4	$ 275.5