Fair Value Measurements - Schedule of Derivative Assets and Liabilities, Including Receivables and Payables (Details) - Fair Value, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	$ (12.1)	$ (17.0)	$ (15.9)
Derivative Assets	$ 127.9	$ 131.6	$ 126.1
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Investments	Other Investments	Other Investments
Effects of netting, liability	$ (22.3)	$ (23.0)	$ (23.1)
Derivative Liabilities	$ 32.9	$ 38.6	$ 27.9
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other noncurrent liabilities	Other noncurrent liabilities	Other noncurrent liabilities
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	$ 86.5	$ 93.8	$ 81.8
Derivative liability fair value gross liability	35.3	44.9	21.9
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	53.5	54.8	60.2
Derivative liability fair value gross liability	19.9	16.7	29.1
U S Stock And Bond Mutual Funds [Member] | Gas Utility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	27.2	27.5	24.7
U S Stock And Bond Mutual Funds [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	23.6	27.0	20.8
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	27.2	27.5	24.7
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	23.6	27.0	20.8
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Gas Utility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	(1.6)	(2.9)	(5.9)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(3.0)	(1.3)	(6.8)
Derivative Liabilities	14.8	24.6	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	(8.7)	(11.4)	(8.8)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(17.5)	(19.0)	(15.1)
Derivative Liabilities	0.0	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Utility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	1.6	2.9	5.9
Derivative liability fair value gross liability	17.8	25.9	6.8
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	8.7	11.4	8.8
Derivative liability fair value gross liability	17.5	19.0	15.1
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Utility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0	0.0
N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0	0.0
Natural Gas Commodity Contracts [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	(1.8)	(2.7)	(1.2)
Derivative Assets	37.7	42.1	42.9
Effects of netting, liability	(1.8)	(2.7)	(1.2)
Derivative Liabilities	18.1	14.0	27.9
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 1 [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Derivative liability fair value gross liability	0.0	0.0	0.0
Natural Gas Commodity Contracts [Member] | Fair Value, Inputs, Level 2 [Member] | Gas Marketing [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	39.5	44.8	44.1
Derivative liability fair value gross liability	19.9	16.7	29.1
U.S. Bonds [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	22.9	23.5	15.6
U.S. Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	22.9	23.5	15.6
U.S. Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Global Bonds [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	2.5	1.5	6.0
Global Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	2.5	1.5	6.0
Global Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Interest Rate Swap [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	14.0	10.0	16.1
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member] | Other Operating Segment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	14.0	10.0	16.1
Spire Missouri [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	(1.6)	(2.9)	(5.9)
Derivative Assets	$ 27.2	$ 27.5	$ 24.7
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other Property and Investments	Other Property and Investments	Other Property and Investments
Spire Missouri [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	$ 28.8	$ 30.4	$ 30.6
Spire Missouri [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	0.0	0.0	0.0
Derivative Assets	27.2	27.5	24.7
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	27.2	27.5	24.7
Spire Missouri [Member] | U S Stock And Bond Mutual Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effects of netting, asset	(1.6)	(2.9)	(5.9)
Derivative Assets	0.0	0.0	0.0
Effects of netting, liability	(3.0)	(1.3)	(6.8)
Derivative Liabilities	14.8	24.6	0.0
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	1.6	2.9	5.9
Derivative liability fair value gross liability	17.8	25.9	6.8
Spire Missouri [Member] | N Y M E X And I C E Natural Gas Contracts [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset fair value gross asset	0.0	0.0	0.0
Derivative liability fair value gross liability	$ 0.0	$ 0.0	$ 0.0