v3.25.2
Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Spire Missouri [Member]        
Loss on lump-sum settlements     $ 1.0  
Pension Plan [Member]        
Service cost – benefits earned during the period $ 4.4 $ 3.9 13.2 $ 11.8
Interest cost on projected benefit obligation 5.8 6.6 17.6 20.0
Expected return on plan assets (6.4) (6.3) (19.3) (18.7)
Amortization of prior service cost (credit) (1.2) (1.1) (3.5) (3.4)
Amortization of actuarial gain (loss) 1.6 1.6 4.8 4.8
Loss on lump-sum settlements 0.0 1.0 0.0 1.0
Subtotal 4.2 5.7 12.8 15.5
Regulatory adjustment 9.2 8.8 27.3 26.3
Net pension cost 13.4 14.5 40.1 41.8
Pension Plan [Member] | Spire Missouri [Member]        
Service cost – benefits earned during the period 2.9 2.6 8.6 7.8
Interest cost on projected benefit obligation 3.9 4.7 11.9 14.2
Expected return on plan assets (4.4) (4.3) (13.2) (12.8)
Amortization of prior service cost (credit) (0.5) (0.5) (1.5) (1.5)
Amortization of actuarial gain (loss) 1.0 1.5 3.0 4.4
Loss on lump-sum settlements 0.0 1.0 0.0 1.0
Subtotal 2.9 5.0 8.8 13.1
Regulatory adjustment 7.4 6.2 22.2 18.8
Net pension cost 10.3 11.2 31.0 31.9
Pension Plan [Member] | Spire Alabama Inc [Member]        
Service cost – benefits earned during the period 1.4 1.2 4.1 3.5
Interest cost on projected benefit obligation 1.3 1.3 3.8 3.8
Expected return on plan assets (1.3) (1.2) (3.9) (3.6)
Amortization of prior service cost (credit) (0.7) (0.6) (1.9) (1.8)
Amortization of actuarial gain (loss) 0.6 0.2 1.8 0.7
Subtotal 1.3 0.9 3.9 2.6
Regulatory adjustment 1.5 2.3 4.4 6.8
Net pension cost 2.8 3.2 8.3 9.4
Other Postretirement Benefits Plan [Member]        
Service cost – benefits earned during the period 1.1 1.2 3.2 3.3
Interest cost on projected benefit obligation 1.7 2.1 5.3 6.6
Expected return on plan assets (4.1) (3.7) (12.5) (11.9)
Amortization of prior service cost (credit) 0.1 0.0 0.1 0.2
Amortization of actuarial gain (loss) (1.5) (1.2) (4.5) (3.3)
Subtotal (2.7) (1.6) (8.4) (5.1)
Regulatory adjustment 1.1 0.1 3.4 0.6
Net pension cost (1.6) (1.5) (5.0) (4.5)
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]        
Service cost – benefits earned during the period 0.9 0.9 2.6 2.7
Interest cost on projected benefit obligation 1.2 1.6 3.8 4.9
Expected return on plan assets (2.7) (2.5) (8.2) (7.9)
Amortization of prior service cost (credit) 0.1 0.1 0.2 0.4
Amortization of actuarial gain (loss) (1.3) (1.0) (3.9) (2.8)
Subtotal (1.8) (0.9) (5.5) (2.7)
Regulatory adjustment 1.5 0.6 4.7 1.9
Net pension cost (0.3) (0.3) (0.8) (0.8)
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]        
Service cost – benefits earned during the period 0.2 0.2 0.6 0.5
Interest cost on projected benefit obligation 0.4 0.5 1.4 1.6
Expected return on plan assets (1.3) (1.2) (4.0) (3.8)
Amortization of prior service cost (credit) 0.0 (0.1) (0.1) (0.2)
Amortization of actuarial gain (loss) 0.0 0.0 (0.1) (0.1)
Subtotal (0.7) (0.6) (2.2) (2.0)
Regulatory adjustment (0.5) (0.5) (1.4) (1.4)
Net pension cost $ (1.2) $ (1.1) $ (3.6) $ (3.4)