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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Sep. 30, 2017
Mar. 31, 2017
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 673.6 $ 791.1 $ 827.7
Total Regulatory Assets 771.3 966.6 953.5
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 353.1 157.2 144.1
Total Regulatory Liabilities 375.3 179.8 175.4
Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 1.4 1.1
Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 13.5
Deferred taxes due to customers      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 170.7 0.0 0.0
Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 30.8 32.2 27.6
Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 5.3 8.2 8.1
Regulatory Liabilities: Non-current 0.6 4.1 4.9
Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 71.0 83.8 75.3
Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.5 1.0 1.6
Regulatory Liabilities: Non-current 50.3 1.9 2.0
Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 15.4 12.0 7.0
Regulatory Liabilities: Non-current 29.7 35.2 34.3
Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 22.2 22.6 31.3
Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 353.1 157.2 144.1
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 113.0 170.5 159.7
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 43.0 42.2 41.7
Regulatory Assets: Non-current 358.2 404.7 461.8
Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 125.0 123.3 133.9
Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 32.6 102.6 61.0
Regulatory Assets: Non-current 0.0 9.9 1.1
Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 31.3 29.0 27.0
Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 22.1 30.7 23.1
Regulatory Assets: Non-current 46.1 53.7 44.2
Total Current Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 97.7 175.5 125.8
Total Noncurrent Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 673.6 791.1 827.7
Spire Missouri      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 446.6 557.8 569.4
Total Regulatory Assets 498.3 653.4 624.7
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 272.5 81.2 84.5
Total Regulatory Liabilities 275.2 83.9 87.2
Spire Missouri | Deferred taxes due to customers      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 152.6 0.0 0.0
Spire Missouri | Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 48.7 54.5 55.6
Spire Missouri | Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 50.3 1.9 2.0
Spire Missouri | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 2.7 2.7
Regulatory Liabilities: Non-current 20.9 24.8 26.9
Spire Missouri | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 2.7 2.7
Spire Missouri | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 272.5 81.2 84.5
Spire Missouri | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 111.4 170.5 159.7
Spire Missouri | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 34.9 34.9 34.8
Regulatory Assets: Non-current 279.0 322.7 358.3
Spire Missouri | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 13.5 57.4 17.1
Regulatory Assets: Non-current 0.0 9.9 1.1
Spire Missouri | Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 31.3 29.0 27.0
Spire Missouri | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 3.3 3.3 3.4
Regulatory Assets: Non-current 24.9 25.7 23.3
Spire Missouri | Total Current Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 51.7 95.6 55.3
Spire Missouri | Total Noncurrent Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 446.6 557.8 569.4
Spire Alabama      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 198.9 197.0 229.3
Total Regulatory Assets 231.0 261.6 284.4
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 35.0 39.6 35.9
Total Regulatory Liabilities 45.8 51.6 61.0
Spire Alabama | Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 1.4 1.1
Spire Alabama | Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 13.5
Spire Alabama | Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 30.8 32.2 27.6
Spire Alabama | Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 5.3 8.2 8.1
Regulatory Liabilities: Non-current 0.6 4.1 4.9
Spire Alabama | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 5.5 2.4 2.4
Regulatory Liabilities: Non-current 3.6 3.3 3.4
Spire Alabama | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 10.8 12.0 25.1
Spire Alabama | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 35.0 39.6 35.9
Spire Alabama | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 7.1 7.2 6.8
Regulatory Assets: Non-current 71.2 72.6 94.4
Spire Alabama | Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 125.0 123.3 133.9
Spire Alabama | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 19.0 45.2 43.9
Spire Alabama | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 6.0 12.2 4.4
Regulatory Assets: Non-current 2.7 1.1 1.0
Spire Alabama | Total Current Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 32.1 64.6 55.1
Spire Alabama | Total Noncurrent Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 198.9 $ 197.0 $ 229.3