Note 13 - Pension Plans and Other Postretirement Benefits - Amounts Recognized in AOCI Not Yet Recognized As Components of Net Periodic Pension Costs (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Spire Missouri [Member]
Net actuarial loss		$ 92.5
Prior service credit		(16.2)
Subtotal		76.3
Adjustments for amounts included in regulatory assets		(73.3)
Total		3.0
Spire Alabama Inc [Member]
Net actuarial loss		40.2
Prior service credit		(13.9)
Subtotal		26.3
Adjustments for amounts included in regulatory assets		(26.3)
Total		0.0
Pension Plan [Member]
Net actuarial loss	$ 119.7	130.1
Prior service credit	(26.4)	(31.2)
Subtotal	93.3	98.9
Adjustments for amounts included in regulatory assets	(90.8)	(95.9)
Total	2.5	3.0
Pension Plan [Member] | Spire Missouri [Member]
Net actuarial loss	72.9
Prior service credit	(14.0)
Subtotal	58.9
Adjustments for amounts included in regulatory assets	(56.4)
Total	2.5
Pension Plan [Member] | Spire Alabama Inc [Member]
Net actuarial loss	47.7
Prior service credit	(11.5)
Subtotal	36.2
Adjustments for amounts included in regulatory assets	(36.2)
Total	0.0
Other Postretirement Benefits Plan [Member]
Net actuarial loss	(150.0)	(103.3)
Prior service credit	2.0	7.0
Subtotal	(148.0)	(96.3)
Adjustments for amounts included in regulatory assets	148.0	96.3
Total	0.0	0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Net actuarial loss	(127.5)	(92.3)
Prior service credit	2.1	7.4
Subtotal	(125.4)	(84.9)
Adjustments for amounts included in regulatory assets	125.4	84.9
Total	0.0	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Net actuarial loss	(15.4)	(6.1)
Prior service credit	(0.1)	(0.4)
Subtotal	(15.5)	(6.5)
Adjustments for amounts included in regulatory assets	15.5	6.5
Total	$ 0.0	$ 0.0