v3.26.1
Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period $ 4.2 $ 4.4 $ 12.8 $ 13.2
Interest cost on projected benefit obligation 6.0 5.8 18.3 17.6
Expected return on plan assets (6.9) (6.4) (20.6) (19.3)
Amortization of prior service cost (credit) (0.9) (1.2) (3.2) (3.5)
Amortization of actuarial gain (loss) 1.4 1.6 4.3 4.8
Subtotal 3.8 4.2 11.6 12.8
Regulatory adjustment 3.8 9.2 16.2 27.3
Net pension cost 7.6 13.4 27.8 40.1
Pension Plan [Member] | Spire Missouri [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 2.9 2.9 8.7 8.6
Interest cost on projected benefit obligation 4.1 3.9 12.4 11.9
Expected return on plan assets (4.4) (4.4) (13.2) (13.2)
Amortization of prior service cost (credit) (0.3) (0.5) (1.3) (1.5)
Amortization of actuarial gain (loss) 0.9 1.0 2.8 3.0
Subtotal 3.2 2.9 9.4 8.8
Regulatory adjustment 2.8 7.4 14.1 22.2
Net pension cost 6.0 10.3 23.5 31.0
Pension Plan [Member] | Spire Alabama Inc [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 1.2 1.4 3.6 4.1
Interest cost on projected benefit obligation 1.3 1.3 3.9 3.8
Expected return on plan assets (1.6) (1.3) (4.8) (3.9)
Amortization of prior service cost (credit) (0.6) (0.7) (1.8) (1.9)
Amortization of actuarial gain (loss) 0.5 0.6 1.5 1.8
Subtotal 0.8 1.3 2.4 3.9
Regulatory adjustment 0.5 1.5 1.4 4.4
Net pension cost 1.3 2.8 3.8 8.3
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 1.0 1.1 2.9 3.2
Interest cost on projected benefit obligation 1.7 1.7 5.2 5.3
Expected return on plan assets (4.6) (4.1) (13.5) (12.5)
Amortization of prior service cost (credit) 0.3 0.1 0.6 0.1
Amortization of actuarial gain (loss) (1.9) (1.5) (5.7) (4.5)
Subtotal (3.5) (2.7) (10.5) (8.4)
Regulatory adjustment 2.0 1.1 6.0 3.4
Net pension cost (1.5) (1.6) (4.5) (5.0)
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost – benefits earned during the period 0.8 0.9 2.3 2.6
Interest cost on projected benefit obligation 1.2 1.2 3.7 3.8
Expected return on plan assets (3.0) (2.7) (8.9) (8.2)
Amortization of prior service cost (credit) 0.2 0.1 0.5 0.2
Amortization of actuarial gain (loss) (1.6) (1.3) (5.0) (3.9)
Subtotal (2.4) (1.8) (7.4) (5.5)
Regulatory adjustment 2.4 1.5 7.3 4.7
Net pension cost 0.0 (0.3) (0.1) (0.8)
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 0.6 0.6
Interest cost on projected benefit obligation 0.5 0.4 1.5 1.4
Expected return on plan assets (1.4) (1.3) (4.2) (4.0)
Amortization of prior service cost (credit) 0.1 0.0 0.1 (0.1)
Amortization of actuarial gain (loss) (0.1) 0.0 (0.2) (0.1)
Subtotal (0.7) (0.7) (2.2) (2.2)
Regulatory adjustment (0.5) (0.5) (1.4) (1.4)
Net pension cost $ (1.2) $ (1.2) $ (3.6) $ (3.6)