Note 13 - Pension Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Noncurrent liabilities	$ (146.7)	$ (137.3)
Spire Missouri [Member]
Noncurrent liabilities	(110.0)	(103.3)
Spire Alabama Inc [Member]
Noncurrent liabilities	(28.0)	(27.2)
Pension Plan [Member]
Current liabilities	(1.0)	(1.6)
Noncurrent liabilities	(93.9)	(102.4)
Total	(94.9)	(104.0)
Pension Plan [Member] | Spire Missouri [Member]
Current liabilities	(1.0)	(1.6)
Noncurrent liabilities	(58.9)	(69.6)
Total	(59.9)	(71.2)
Pension Plan [Member] | Spire Alabama Inc [Member]
Current liabilities	0.0	0.0
Noncurrent liabilities	(28.0)	(27.0)
Total	(28.0)	(27.0)
Other Postretirement Benefits Plan [Member]
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(51.0)	(36.6)
Noncurrent assets	248.3	180.7
Total	196.9	143.7
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(51.0)	(36.6)
Noncurrent assets	173.9	120.5
Total	122.5	83.5
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Noncurrent assets	68.5	56.9
Total	$ 68.5	$ 56.9