Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 7.4	$ 5.8	$ 5.8
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.7	2.6	3.7
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	477.6	743.2	434.0
Long-term debt, including current portion	2,875.5	2,373.4	2,363.1
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	218.3	386.4	281.5
Long-term debt, including current portion	1,303.2	1,065.2	1,027.6
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	85.6	128.7	79.6
Long-term debt, including current portion	572.3	474.8	450.2
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.4	5.8	5.8
Notes payable	477.6	743.2	434.0
Long-term debt, including current portion	2,483.7	2,122.6	2,207.3
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.7	2.6	3.7
Notes payable – associated companies	218.3	386.4	281.5
Long-term debt, including current portion	1,091.9	925.0	924.8
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	85.6	128.7	79.6
Long-term debt, including current portion	471.8	412.2	412.1
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.4	5.8	5.8
Notes payable	477.6	743.2	434.0
Long-term debt, including current portion	2,875.5	2,373.4	2,363.1
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.7	2.6	3.7
Notes payable – associated companies	218.3	386.4	281.5
Long-term debt, including current portion	1,303.2	1,065.2	1,027.6
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – associated companies	85.6	128.7	79.6
Long-term debt, including current portion	$ 572.3	$ 474.8	$ 450.2