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Pension Plans and Other Postretirement Benefits - Pre-tax Amounts Expected to Amortized From Accumulated Other Comprehensive Loss Into Net Periodic Pension Cost (Details)
$ in Millions
12 Months Ended
Sep. 30, 2019
USD ($)
Pension Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial (gain) loss $ 14.7
Amortization of prior service (credit) cost (2.5)
Subtotal 12.2
Regulatory adjustment (12.0)
Total 0.2
Pension Plans | Spire Missouri  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial (gain) loss 11.6
Amortization of prior service (credit) cost 0.1
Subtotal 11.7
Regulatory adjustment (11.5)
Total 0.2
Pension Plans | Spire Alabama  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial (gain) loss 3.1
Amortization of prior service (credit) cost (2.4)
Subtotal 0.7
Regulatory adjustment (0.7)
Total 0.0
Other Postretirement Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial (gain) loss (2.0)
Amortization of prior service (credit) cost (0.5)
Subtotal (2.5)
Regulatory adjustment 2.5
Other Postretirement Plans | Spire Missouri  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial (gain) loss (2.0)
Amortization of prior service (credit) cost (0.2)
Subtotal (2.2)
Regulatory adjustment 2.2
Other Postretirement Plans | Spire Alabama  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of prior service (credit) cost (0.3)
Subtotal (0.3)
Regulatory adjustment $ 0.3