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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 716.6 $ 791.1 $ 786.4
Total Regulatory Assets 865.9 966.6 920.9
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 335.1 157.2 132.1
Total Regulatory Liabilities 356.6 179.8 176.3
Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.0 1.4 3.8
Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 22.0
Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 31.5 32.2 28.3
Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 7.9 8.2 9.0
Regulatory Liabilities: Non-current 3.8 4.1 8.9
Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.0 1.0 1.4
Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 11.6 12.0 8.0
Regulatory Liabilities: Non-current 40.7 37.1 20.2
Deferred taxes due to customers      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 177.4 0.0 0.0
Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 81.7 83.8 74.7
Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 21.5 22.6 44.2
Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 335.1 157.2 132.1
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 43.0 42.2 63.2
Regulatory Assets: Non-current 394.8 404.7 439.2
Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 77.9 102.6 52.2
Regulatory Assets: Non-current 0.0 9.9 4.7
Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 28.4 30.7 19.1
Regulatory Assets: Non-current 54.8 53.7 29.4
Total Current Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 149.3 175.5 134.5
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 113.1 170.5 155.5
Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 123.9 123.3 131.6
Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 30.0 29.0 26.0
Total Noncurrent Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 716.6 791.1 786.4
Spire Missouri      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 484.1 557.8 543.4
Total Regulatory Assets 560.8 653.4 636.9
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 241.2 81.2 67.3
Total Regulatory Liabilities 243.9 83.9 70.0
Spire Missouri | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 2.7 2.7
Regulatory Liabilities: Non-current 30.0 26.7 12.5
Spire Missouri | Deferred taxes due to customers      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 159.2 0.0 0.0
Spire Missouri | Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 52.0 54.5 54.8
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 241.2 81.2 67.3
Spire Missouri | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 34.9 34.9 56.3
Regulatory Assets: Non-current 315.8 322.7 333.3
Spire Missouri | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 38.5 57.4 33.8
Regulatory Assets: Non-current 0.0 9.9 4.7
Spire Missouri | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 3.3 3.3 3.4
Regulatory Assets: Non-current 25.2 25.7 23.9
Spire Missouri | Total Current Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 76.7 95.6 93.5
Spire Missouri | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 113.1 170.5 155.5
Spire Missouri | Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 30.0 29.0 26.0
Spire Missouri | Total Noncurrent Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 484.1 557.8 543.4
Spire Alabama      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 197.4 197.0 229.5
Total Regulatory Assets 255.4 261.6 261.0
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 39.0 39.6 40.6
Total Regulatory Liabilities 50.3 51.6 78.0
Spire Alabama | Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.0 1.4 3.8
Spire Alabama | Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 22.0
Spire Alabama | Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 31.5 32.2 28.3
Spire Alabama | Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 7.9 8.2 9.0
Regulatory Liabilities: Non-current 3.9 4.1 8.9
Spire Alabama | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.4 2.4 2.6
Regulatory Liabilities: Non-current 3.6 3.3 3.4
Spire Alabama | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 11.3 12.0 37.4
Spire Alabama | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 39.0 39.6 40.6
Spire Alabama | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 7.2 7.2 6.8
Regulatory Assets: Non-current 70.8 72.6 96.8
Spire Alabama | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 39.4 45.2 17.1
Spire Alabama | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 11.4 12.2 7.6
Regulatory Assets: Non-current 2.7 1.1 1.1
Spire Alabama | Total Current Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 58.0 64.6 31.5
Spire Alabama | Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 123.9 123.3 131.6
Spire Alabama | Total Noncurrent Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 197.4 $ 197.0 $ 229.5