Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4.3	$ 4.1
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	672.0	648.0
Long-term debt, including current portion	3,375.9	2,908.6
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	250.0
Notes payable - associated companies	240.9	301.2
Long-term debt, including current portion	1,540.4	1,313.5
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	49.0	121.3
Long-term debt, including current portion	707.5	576.9
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.3	4.1
Notes payable	672.0	648.0
Long-term debt, including current portion	2,994.9	2,484.1
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	250.0
Notes payable - associated companies	240.9	301.2
Long-term debt, including current portion	1,338.4	1,092.0
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	49.0	121.3
Long-term debt, including current portion	621.2	471.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.3	4.1
Notes payable	672.0	648.0
Long-term debt, including current portion	3,375.9	2,908.6
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	250.0
Notes payable - associated companies	240.9	301.2
Long-term debt, including current portion	1,540.4	1,313.5
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable - associated companies	49.0	121.3
Long-term debt, including current portion	$ 707.5	$ 576.9