Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 23.9	$ 4.1	$ 7.4
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.3		0.7
Quoted Prices in Active Markets (Level 1) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	9.5
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	461.0	648.0	477.6
Long-term debt, including current portion	3,440.2	2,908.6	2,875.5
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	217.5	301.2	218.3
Long-term debt, including current portion	1,548.8	1,313.5	1,303.2
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies		121.3	85.6
Long-term debt, including current portion	710.5	576.9	572.3
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	23.9	4.1	7.4
Notes payable	461.0	648.0	477.6
Long-term debt, including current portion	3,049.8	2,484.1	2,483.7
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.3		0.7
Notes payable	250.0
Notes payable – associated companies	217.5	301.2	218.3
Long-term debt, including current portion	1,338.6	1,092.0	1,091.9
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	9.5
Notes payable – associated companies		121.3	85.6
Long-term debt, including current portion	621.2	471.8	471.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	23.9	4.1	7.4
Notes payable	461.0	648.0	477.6
Long-term debt, including current portion	3,440.2	2,908.6	2,875.5
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.3		0.7
Notes payable	250.0
Notes payable – associated companies	217.5	301.2	218.3
Long-term debt, including current portion	1,548.8	1,313.5	1,303.2
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	9.5
Notes payable – associated companies		121.3	85.6
Long-term debt, including current portion	$ 710.5	$ 576.9	$ 572.3