Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018	Jun. 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	$ 6.9
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.7	2.0	3.5
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	434.0	553.6	191.0
Long-term debt, including current portion	2,363.1	2,074.0	2,173.7
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	1,027.6	906.6	1,005.0
Significant Observable Inputs (Level 2) | Spire Missouri | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	281.5	345.3	128.6
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	450.2	321.7	317.0
Significant Observable Inputs (Level 2) | Spire Alabama | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	79.6	142.5	69.6
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.8	4.4	6.9
Notes payable	434.0	553.6	191.0
Long-term debt, including current portion	2,207.3	2,075.6	2,180.0
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.7	2.0	3.5
Long-term debt, including current portion	924.8	874.4	974.2
Carrying Amount | Spire Missouri | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	281.5	345.3	128.6
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	412.1	322.6	322.5
Carrying Amount | Spire Alabama | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	79.6	142.5	69.6
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.8	4.4	6.9
Notes payable	434.0	553.6	191.0
Long-term debt, including current portion	2,363.1	2,074.0	2,173.7
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.7	2.0	3.5
Long-term debt, including current portion	1,027.6	906.6	1,005.0
Fair Value | Spire Missouri | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	281.5	345.3	128.6
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	450.2	321.7	317.0
Fair Value | Spire Alabama | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	$ 79.6	$ 142.5	$ 69.6