v3.26.1
Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2026
Mar. 31, 2025
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period $ 4.3 $ 4.3 $ 8.6 $ 8.8
Interest cost on projected benefit obligation 6.2 5.8 12.3 11.8
Expected return on plan assets (6.8) (6.4) (13.7) (12.9)
Amortization of prior service cost (credit) (1.3) (1.2) (2.3) (2.3)
Amortization of actuarial gain (loss) 1.5 1.6 2.9 3.2
Subtotal 3.9 4.1 7.8 8.6
Regulatory adjustment 6.6 9.2 12.4 18.1
Net pension cost 10.5 13.3 20.2 26.7
Pension Plan [Member] | Spire Missouri [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 2.9 2.8 5.8 5.7
Interest cost on projected benefit obligation 4.2 4.0 8.3 8.0
Expected return on plan assets (4.4) (4.3) (8.8) (8.8)
Amortization of prior service cost (credit) (0.6) (0.5) (1.0) (1.0)
Amortization of actuarial gain (loss) 1.0 1.0 1.9 2.0
Subtotal 3.1 3.0 6.2 5.9
Regulatory adjustment 6.2 7.4 11.3 14.8
Net pension cost 9.3 10.4 17.5 20.7
Pension Plan [Member] | Spire Alabama Inc [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 1.2 1.3 2.4 2.7
Interest cost on projected benefit obligation 1.3 1.2 2.6 2.5
Expected return on plan assets (1.6) (1.3) (3.2) (2.6)
Amortization of prior service cost (credit) (0.6) (0.6) (1.2) (1.2)
Amortization of actuarial gain (loss) 0.5 0.6 1.0 1.2
Subtotal 0.8 1.2 1.6 2.6
Regulatory adjustment 0.4 1.6 0.9 2.9
Net pension cost 1.2 2.8 2.5 5.5
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 0.9 1.0 1.9 2.1
Interest cost on projected benefit obligation 1.8 1.8 3.5 3.6
Expected return on plan assets (4.4) (4.2) (8.9) (8.4)
Amortization of prior service cost (credit) 0.1 0.0 0.3 0.0
Amortization of actuarial gain (loss) (1.8) (1.5) (3.8) (3.0)
Subtotal (3.4) (2.9) (7.0) (5.7)
Regulatory adjustment 2.1 1.2 4.0 2.3
Net pension cost (1.3) (1.7) (3.0) (3.4)
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 0.7 0.8 1.5 1.7
Interest cost on projected benefit obligation 1.3 1.3 2.5 2.6
Expected return on plan assets (2.9) (2.7) (5.9) (5.5)
Amortization of prior service cost (credit) 0.1 0.0 0.3 0.1
Amortization of actuarial gain (loss) (1.7) (1.3) (3.4) (2.6)
Subtotal (2.5) (1.9) (5.0) (3.7)
Regulatory adjustment 2.5 1.6 4.9 3.2
Net pension cost (0.0) (0.3) (0.1) (0.5)
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 0.2 0.2 0.4 0.4
Interest cost on projected benefit obligation 0.5 0.5 1.0 1.0
Expected return on plan assets (1.4) (1.4) (2.8) (2.7)
Amortization of prior service cost (credit) (0.0) (0.0) (0.0) (0.1)
Amortization of actuarial gain (loss) 0.0 (0.1) (0.1) (0.1)
Subtotal (0.7) (0.8) (1.5) (1.5)
Regulatory adjustment (0.4) (0.4) (0.9) (0.9)
Net pension cost $ (1.1) $ (1.2) $ (2.4) $ (2.4)