FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 17.8	$ 7.4	$ 19.6	$ 5.2
Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	10.2	2.5	3.7	2.1
Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.1	0.0	$ 0.0
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	17.8	7.4	19.6
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	10.2	2.5	3.7
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	0.0
Long-term debt, including current portion	0.0	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	0.0
Short-term debt	391.7	477.3	567.4
Long-term debt, including current portion	2,241.3	2,210.3	1,993.9
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	0.0
Short-term debt	175.8	203.0	282.2
Long-term debt, including current portion	1,033.6	1,056.9	870.1
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	82.3	169.9	109.3
Long-term debt, including current portion	332.7	269.4	261.6
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	17.8	7.4	19.6
Short-term debt	391.7	477.3	567.4
Long-term debt, including current portion	2,179.4	2,095.0	1,925.3
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	10.2	2.5	3.7
Short-term debt	175.8	203.0	282.2
Long-term debt, including current portion	974.0	973.9	804.3
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1
Short-term debt	82.3	169.9	109.3
Long-term debt, including current portion	322.5	247.8	247.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	17.8	7.4	19.6
Short-term debt	391.7	477.3	567.4
Long-term debt, including current portion	2,241.3	2,210.3	1,993.9
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	10.2	2.5	3.7
Short-term debt	175.8	203.0	282.2
Long-term debt, including current portion	1,033.6	1,056.9	870.1
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1
Short-term debt	82.3	169.9	109.3
Long-term debt, including current portion	$ 332.7	$ 269.4	$ 261.6