Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 8.2	$ 4.3	$ 3.5
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	846.0	672.0	696.1
Long-term debt, including current portion	3,594.4	3,375.9	3,119.5
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	180.4	240.9	393.7
Long-term debt, including current portion	1,830.5	1,540.4	1,329.0
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	173.7	49.0	27.0
Long-term debt, including current portion	653.6	707.5	742.9
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.2	4.3	3.5
Notes payable	846.0	672.0	696.1
Long-term debt, including current portion	3,238.0	2,994.9	2,628.4
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	250.0	250.0
Notes payable – associated companies	180.4	240.9	393.7
Long-term debt, including current portion	1,637.0	1,338.4	1,092.2
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	173.7	49.0	27.0
Long-term debt, including current portion	571.3	621.2	621.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.2	4.3	3.5
Notes payable	846.0	672.0	696.1
Long-term debt, including current portion	3,594.4	3,375.9	3,119.5
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	250.0	250.0
Notes payable – associated companies	180.4	240.9	393.7
Long-term debt, including current portion	1,830.5	1,540.4	1,329.0
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	173.7	49.0	27.0
Long-term debt, including current portion	$ 653.6	$ 707.5	$ 742.9