Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4.1	$ 5.8
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		2.6
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	648.0	743.2
Long-term debt, including current portion	2,908.6	2,373.4
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	301.2	386.4
Long-term debt, including current portion	1,313.5	1,065.2
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	121.3	128.7
Long-term debt, including current portion	576.9	474.8
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.1	5.8
Notes payable	648.0	743.2
Long-term debt, including current portion	2,484.1	2,122.6
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		2.6
Notes payable	301.2	386.4
Long-term debt, including current portion	1,092.0	925.0
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	121.3	128.7
Long-term debt, including current portion	471.8	412.2
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.1	5.8
Notes payable	648.0	743.2
Long-term debt, including current portion	2,908.6	2,373.4
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		2.6
Notes payable	301.2	386.4
Long-term debt, including current portion	1,313.5	1,065.2
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	121.3	128.7
Long-term debt, including current portion	$ 576.9	$ 474.8