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Revenue - Schedule of Operating Revenue Information (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Disaggregation Of Revenue [Line Items]        
Total operating revenues $ 327.8 $ 321.1 $ 1,945.3 $ 1,603.5
Spire Missouri        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 193.3 207.3 1,335.8 1,033.1
Changes in accrued revenue under alternative revenue programs (1.2) (3.4) 2.7 2.3
Total operating revenues 192.1 203.9 1,338.5 1,035.4
Spire Missouri | Residential        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 144.4 153.2 772.8 760.2
Spire Missouri | Commercial & Industrial        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 35.0 35.9 408.9 215.9
Spire Missouri | Transportation        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 7.4 7.4 26.3 26.1
Spire Missouri | Off-system & Other Incentive        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 4.5 7.6 140.4 26.3
Spire Missouri | Other Customer Revenue        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 2.0 3.2 (12.6) 4.6
Spire Alabama        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 89.6 80.1 429.2 395.2
Changes in accrued revenue under alternative revenue programs 3.7 1.1 0.3 (2.3)
Total operating revenues 93.3 81.2 429.5 392.9
Spire Alabama | Residential        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 45.6 44.7 259.6 239.5
Spire Alabama | Commercial & Industrial        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 21.3 17.1 98.8 94.6
Spire Alabama | Transportation        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 18.7 16.2 60.9 55.7
Spire Alabama | Off-system & Other Incentive        
Disaggregation Of Revenue [Line Items]        
Total operating revenues 2.8 1.7 6.7 1.7
Spire Alabama | Other Customer Revenue        
Disaggregation Of Revenue [Line Items]        
Total operating revenues $ 1.2 $ 0.4 $ 3.2 $ 3.7