Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 5.8	$ 4.4
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.0
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	743.2	553.6
Long-term debt, including current portion	2,373.4	2,074.0
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	386.4	345.3
Long-term debt, including current portion	1,065.2	906.6
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	128.7	142.5
Long-term debt, including current portion	474.8	321.7
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.8	4.4
Notes payable	743.2	553.6
Long-term debt, including current portion	2,122.6	2,075.6
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.0
Notes payable	386.4	345.3
Long-term debt, including current portion	925.0	874.4
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	128.7	142.5
Long-term debt, including current portion	412.2	322.6
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.8	4.4
Notes payable	743.2	553.6
Long-term debt, including current portion	2,373.4	2,074.0
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.0
Notes payable	386.4	345.3
Long-term debt, including current portion	1,065.2	906.6
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	128.7	142.5
Long-term debt, including current portion	$ 474.8	$ 321.7