Fair Value of Financial Instruments - Schedule of Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 11.5	$ 4.5	$ 4.8
Notes payable	1,158.0	947.0	1,047.5
Long-term debt, including current portion	3,740.2	3,746.4	3,704.8
Reported Value Measurement [Member] | Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	1,803.6	1,803.4	1,785.8
Notes payable – associated companies	580.9	495.3	633.3
Reported Value Measurement [Member] | Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	67.0	1.5
Long-term debt, including current portion	746.4	746.3	746.0
Notes payable – associated companies		48.4	128.5
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	11.5	4.5	4.8
Notes payable	1,158.0	947.0	1,047.5
Long-term debt, including current portion	3,435.3	3,600.3	3,481.5
Estimate of Fair Value Measurement [Member] | Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	1,646.3	1,736.9	1,689.5
Notes payable – associated companies	580.9	495.3	633.3
Estimate of Fair Value Measurement [Member] | Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	67.0	1.5
Long-term debt, including current portion	677.4	711.8	701.7
Notes payable – associated companies		48.4	128.5
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	11.5	4.5	4.8
Notes payable	0.0	0.0	0.0
Long-term debt, including current portion	0.0	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	0.0	0.0	0.0
Notes payable – associated companies	0.0	0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	1.5
Long-term debt, including current portion	0.0	0.0	0.0
Notes payable – associated companies		0.0	0.0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Notes payable	1,158.0	947.0	1,047.5
Long-term debt, including current portion	3,435.3	3,600.3	3,481.5
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion	1,646.3	1,736.9	1,689.5
Notes payable – associated companies	580.9	495.3	633.3
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	67.0	0.0
Long-term debt, including current portion	$ 677.4	711.8	701.7
Notes payable – associated companies		$ 48.4	$ 128.5