Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021	Mar. 31, 2021
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 8.3	$ 4.3	$ 104.0
Quoted Prices in Active Markets (Level 1) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2		18.2
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	607.1	672.0	653.5
Long-term debt, including current portion	3,299.1	3,375.9	3,090.9
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable		250.0	250.0
Notes payable – associated companies	277.2	240.9	369.4
Long-term debt, including current portion	1,691.9	1,540.4	1,254.2
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	156.6	49.0
Long-term debt, including current portion	588.0	707.5	682.3
Notes receivable – associated companies			20.0
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.3	4.3	104.0
Notes payable	607.1	672.0	653.5
Long-term debt, including current portion	3,238.5	2,994.9	2,803.3
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable		250.0	250.0
Notes payable – associated companies	277.2	240.9	369.4
Long-term debt, including current portion	1,637.1	1,338.4	1,092.4
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2		18.2
Notes payable – associated companies	156.6	49.0
Long-term debt, including current portion	571.4	621.2	621.1
Notes receivable – associated companies			20.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.3	4.3	104.0
Notes payable	607.1	672.0	653.5
Long-term debt, including current portion	3,299.1	3,375.9	3,090.9
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable		250.0	250.0
Notes payable – associated companies	277.2	240.9	369.4
Long-term debt, including current portion	1,691.9	1,540.4	1,254.2
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2		18.2
Notes payable – associated companies	156.6	49.0
Long-term debt, including current portion	$ 588.0	$ 707.5	682.3
Notes receivable – associated companies			$ 20.0