Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 8.4	$ 4.4	$ 6.7
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.9	2.0	4.1
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	626.1	553.6	583.6
Long-term debt, including current portion	2,133.9	2,074.0	2,280.1
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	308.0	345.3	275.6
Long-term debt, including current portion	997.3	906.6	1,068.6
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	164.2	142.5	163.1
Long-term debt, including current portion	311.3	321.7	303.5
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.4	4.4	6.7
Short-term debt	626.1	553.6	583.6
Long-term debt, including current portion	2,167.0	2,075.6	2,135.5
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.9	2.0	4.1
Short-term debt	308.0	345.3	275.6
Long-term debt, including current portion	974.5	874.4	974.1
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	164.2	142.5	163.1
Long-term debt, including current portion	322.6	322.6	277.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.4	4.4	6.7
Short-term debt	626.1	553.6	583.6
Long-term debt, including current portion	2,133.9	2,074.0	2,280.1
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.9	2.0	4.1
Short-term debt	308.0	345.3	275.6
Long-term debt, including current portion	997.3	906.6	1,068.6
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	164.2	142.5	163.1
Long-term debt, including current portion	$ 311.3	$ 321.7	$ 303.5