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, 2023	Sep. 30, 2022
Spire Missouri [Member]
Net actuarial loss		$ 93.6
Prior service credit		(18.1)
Subtotal		75.5
Adjustments for amounts included in regulatory assets		(72.3)
Total		3.2
Spire Alabama Inc [Member]
Net actuarial loss		41.2
Prior service credit		(16.3)
Subtotal		24.9
Adjustments for amounts included in regulatory assets		(24.9)
Total		0.0
Pension Plan [Member]
Net actuarial loss	$ 130.1	132.0
Prior service credit	(31.2)	(35.7)
Subtotal	98.9	96.3
Adjustments for amounts included in regulatory assets	(95.9)	(93.1)
Total	3.0	3.2
Pension Plan [Member] | 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
Pension Plan [Member] | 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
Other Postretirement Benefits Plan [Member]
Net actuarial loss	(103.3)	(83.3)
Prior service credit	7.0	7.1
Subtotal	(96.3)	(76.2)
Adjustments for amounts included in regulatory assets	96.3	76.2
Total	0.0	0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Net actuarial loss	(92.3)	(74.2)
Prior service credit	7.4	7.9
Subtotal	(84.9)	(66.3)
Adjustments for amounts included in regulatory assets	84.9	66.3
Total	0.0	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Net actuarial loss	(6.1)	(5.6)
Prior service credit	(0.4)	(0.8)
Subtotal	(6.5)	(6.4)
Adjustments for amounts included in regulatory assets	6.5	6.4
Total	$ 0.0	$ 0.0