Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 108.4	$ 5.8	$ 11.1
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	2.6	6.9
Quoted Prices in Active Markets (Level 1) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1		0.1
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	560.6	743.2	512.0
Long-term debt, including current portion	2,671.0	2,373.4	2,319.9
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	185.1
Notes payable – associated companies	39.6	386.4	232.9
Long-term debt, including current portion	1,231.2	1,065.2	1,036.9
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	50.0		73.7
Notes payable – associated companies	33.0	128.7
Long-term debt, including current portion	535.1	474.8	423.8
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	108.4	5.8	11.1
Notes payable	560.6	743.2	512.0
Long-term debt, including current portion	2,490.2	2,122.6	2,256.9
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.1	2.6	6.9
Notes payable	185.1
Notes payable – associated companies	39.6	386.4	232.9
Long-term debt, including current portion	1,098.7	925.0	974.7
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1		0.1
Notes payable	50.0		73.7
Notes payable – associated companies	33.0	128.7
Long-term debt, including current portion	471.7	412.2	412.1
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	108.4	5.8	11.1
Notes payable	560.6	743.2	512.0
Long-term debt, including current portion	2,671.0	2,373.4	2,319.9
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.1	2.6	6.9
Notes payable	185.1
Notes payable – associated companies	39.6	386.4	232.9
Long-term debt, including current portion	1,231.2	1,065.2	1,036.9
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1		0.1
Notes payable	50.0		73.7
Notes payable – associated companies	33.0	128.7
Long-term debt, including current portion	$ 535.1	$ 474.8	$ 423.8