Pension Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Noncurrent liabilities	$ (74.7)	$ (146.7)
Spire Missouri [Member]
Noncurrent liabilities	(72.4)	(110.0)
Spire Alabama Inc [Member]
Noncurrent liabilities	(2.4)	(28.0)
Pension Plan [Member]
Noncurrent assets	6.2	0.0
Current liabilities	(0.6)	(1.0)
Noncurrent liabilities	(28.1)	(93.9)
Total	(22.5)	(94.9)
Pension Plan [Member] | Spire Missouri [Member]
Noncurrent assets	3.9	0.0
Current liabilities	(0.6)	(1.0)
Noncurrent liabilities	(25.7)	(58.9)
Total	(22.4)	(59.9)
Pension Plan [Member] | Spire Alabama Inc [Member]
Noncurrent assets	0.0	0.0
Current liabilities	0.0	0.0
Noncurrent liabilities	(2.4)	(28.0)
Total	(2.4)	(28.0)
Other Postretirement Benefits Plan [Member]
Noncurrent assets	288.2	248.3
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(46.4)	(51.0)
Total	241.4	196.9
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Noncurrent assets	206.1	173.9
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(46.4)	(51.0)
Total	159.3	122.5
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Noncurrent assets	75.2	68.5
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	$ 75.2	$ 68.5