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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Pre-tax Amortization from AOCI (Details)
$ in Millions
12 Months Ended
Sep. 30, 2017
USD ($)
Pension Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial loss $ 12.6
Amortization of prior service (credit) cost (0.9)
Subtotal 11.7
Regulatory adjustment (11.4)
Total 0.3
Postretirement Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial loss 0.9
Amortization of prior service (credit) cost (0.1)
Subtotal 0.8
Regulatory adjustment (0.8)
Total 0.0
Spire Missouri | Pension Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial loss 10.5
Amortization of prior service (credit) cost 0.9
Subtotal 11.4
Regulatory adjustment (11.1)
Total 0.3
Spire Missouri | Postretirement Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial loss 0.9
Amortization of prior service (credit) cost 0.3
Subtotal 1.2
Regulatory adjustment (1.2)
Total 0.0
Spire Alabama | Pension Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial loss 2.1
Amortization of prior service (credit) cost (1.8)
Subtotal 0.3
Regulatory adjustment (0.3)
Total 0.0
Spire Alabama | Postretirement Plans  
Pre-tax amounts are expected to be amortized from accumulated other comprehensive income [Abstract]  
Amortization of net actuarial loss 0.0
Amortization of prior service (credit) cost (0.4)
Subtotal (0.4)
Regulatory adjustment 0.4
Total $ 0.0