Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 11.1	$ 4.4	$ 17.8
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.9	2.0	10.2
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
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	512.0	553.6	391.7
Long-term debt, including current portion	2,319.9	2,074.0	2,241.3
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	1,036.9	906.6	1,033.6
Significant Observable Inputs (Level 2) | Spire Missouri | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	232.9	345.3	175.8
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	423.8	321.7	332.7
Significant Observable Inputs (Level 2) | Spire Alabama | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	73.7	142.5	82.3
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	11.1	4.4	17.8
Notes payable	512.0	553.6	391.7
Long-term debt, including current portion	2,256.9	2,075.6	2,179.4
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.9	2.0	10.2
Long-term debt, including current portion	974.7	874.4	974.0
Carrying Amount | Spire Missouri | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	232.9	345.3	175.8
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1
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	73.7	142.5	82.3
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	11.1	4.4	17.8
Notes payable	512.0	553.6	391.7
Long-term debt, including current portion	2,319.9	2,074.0	2,241.3
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.9	2.0	10.2
Long-term debt, including current portion	1,036.9	906.6	1,033.6
Fair Value | Spire Missouri | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	232.9	345.3	175.8
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1
Long-term debt, including current portion	423.8	321.7	332.7
Fair Value | Spire Alabama | Affiliated Entity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	$ 73.7	$ 142.5	$ 82.3