FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 6.7	$ 7.4	$ 10.6	$ 5.2
Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.1	2.5	4.0	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	6.7	7.4	10.6
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	4.1	2.5	4.0
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	0.0	0.1	0.0
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	583.6	477.3	506.4
Long-term debt, including current portion	2,280.1	2,210.3	2,258.1
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	0.0
Short-term debt	275.6	203.0	312.9
Long-term debt, including current portion	1,068.6	1,056.9	910.7
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Short-term debt	163.1	169.9	102.5
Long-term debt, including current portion	303.5	269.4	269.3
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.7	7.4	10.6
Short-term debt	583.6	477.3	506.4
Long-term debt, including current portion	2,135.5	2,095.0	2,071.3
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.1	2.5	4.0
Short-term debt	275.6	203.0	312.9
Long-term debt, including current portion	974.1	973.9	804.3
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.1	0.0
Short-term debt	163.1	169.9	102.5
Long-term debt, including current portion	277.8	247.8	247.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.7	7.4	10.6
Short-term debt	583.6	477.3	506.4
Long-term debt, including current portion	2,280.1	2,210.3	2,258.1
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.1	2.5	4.0
Short-term debt	275.6	203.0	312.9
Long-term debt, including current portion	1,068.6	1,056.9	910.7
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.1	0.0
Short-term debt	163.1	169.9	102.5
Long-term debt, including current portion	$ 303.5	$ 269.4	$ 269.3