Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 3.5	$ 4.1	$ 21.5
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			9.3
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	696.1	648.0	518.9
Long-term debt, including current portion	3,119.5	2,908.6	2,765.9
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	393.7	301.2	288.1
Long-term debt, including current portion	1,329.0	1,313.5	1,234.9
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	27.0	121.3	67.1
Long-term debt, including current portion	742.9	576.9	572.8
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.5	4.1	21.5
Notes payable	696.1	648.0	518.9
Long-term debt, including current portion	2,628.4	2,484.1	2,529.7
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			9.3
Notes payable	393.7	301.2	288.1
Long-term debt, including current portion	1,092.2	1,092.0	1,098.6
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	27.0	121.3	67.1
Long-term debt, including current portion	621.0	471.8	511.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.5	4.1	21.5
Notes payable	696.1	648.0	518.9
Long-term debt, including current portion	3,119.5	2,908.6	2,765.9
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			9.3
Notes payable	393.7	301.2	288.1
Long-term debt, including current portion	1,329.0	1,313.5	1,234.9
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	27.0	121.3	67.1
Long-term debt, including current portion	$ 742.9	$ 576.9	$ 572.8