Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 21.5	$ 5.8	$ 8.4
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	9.3	2.6	4.9
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	518.9	743.2	626.1
Long-term debt, including current portion	2,765.9	2,373.4	2,133.9
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	288.1	386.4	308.0
Long-term debt, including current portion	1,234.9	1,065.2	997.3
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	67.1	128.7	164.2
Long-term debt, including current portion	572.8	474.8	311.3
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	21.5	5.8	8.4
Notes payable	518.9	743.2	626.1
Long-term debt, including current portion	2,529.7	2,122.6	2,167.0
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	9.3	2.6	4.9
Notes payable	288.1	386.4	308.0
Long-term debt, including current portion	1,098.6	925.0	974.5
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	67.1	128.7	164.2
Long-term debt, including current portion	511.7	412.2	322.6
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	21.5	5.8	8.4
Notes payable	518.9	743.2	626.1
Long-term debt, including current portion	2,765.9	2,373.4	2,133.9
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	9.3	2.6	4.9
Notes payable	288.1	386.4	308.0
Long-term debt, including current portion	1,234.9	1,065.2	997.3
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	67.1	128.7	164.2
Long-term debt, including current portion	$ 572.8	$ 474.8	$ 311.3