Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 104.0	$ 4.1	$ 108.4
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			5.1
Quoted Prices in Active Markets (Level 1) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	18.2		0.1
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	653.5	648.0	560.6
Long-term debt, including current portion	3,090.9	2,908.6	2,671.0
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	250.0		185.1
Notes payable – associated companies	369.4	301.2	39.6
Long-term debt, including current portion	1,254.2	1,313.5	1,231.2
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable			50.0
Notes payable – associated companies		121.3	33.0
Notes receivable, associated companies	20.0
Long-term debt, including current portion	682.3	576.9	535.1
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	104.0	4.1	108.4
Notes payable	653.5	648.0	560.6
Long-term debt, including current portion	2,803.3	2,484.1	2,490.2
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			5.1
Notes payable	250.0		185.1
Notes payable – associated companies	369.4	301.2	39.6
Long-term debt, including current portion	1,092.4	1,092.0	1,098.7
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	18.2		0.1
Notes payable			50.0
Notes payable – associated companies		121.3	33.0
Notes receivable, associated companies	20.0
Long-term debt, including current portion	621.1	471.8	471.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	104.0	4.1	108.4
Notes payable	653.5	648.0	560.6
Long-term debt, including current portion	3,090.9	2,908.6	2,671.0
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			5.1
Notes payable	250.0		185.1
Notes payable – associated companies	369.4	301.2	39.6
Long-term debt, including current portion	1,254.2	1,313.5	1,231.2
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	18.2		0.1
Notes payable			50.0
Notes payable – associated companies		121.3	33.0
Notes receivable, associated companies	20.0
Long-term debt, including current portion	$ 682.3	$ 576.9	$ 535.1