Note 13 - Pension Plans and Other Postretirement Benefits - PBO, ABO, and Fair Value of Plan Assets for Plans in Excess of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Projected benefit obligation	$ 438.7	$ 464.4
Accumulated benefit obligation	433.1	457.1
Fair value of plan assets	334.7	332.7
Spire Missouri [Member]
Projected benefit obligation	313.2	320.6
Accumulated benefit obligation	309.5	315.0
Fair value of plan assets	242.0	235.9
Spire Alabama Inc [Member]
Projected benefit obligation	80.6	97.8
Accumulated benefit obligation	78.8	96.2
Fair value of plan assets	$ 53.6	$ 57.3