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REGULATORY MATTERS - Schedule of Regulatory Assets Not Earnings a Return (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Sep. 30, 2017
Mar. 31, 2017
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return $ 309.8 $ 380.3 $ 412.5
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return 113.0 170.5 159.7
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return 185.8 198.5 241.0
Other      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return 11.0 11.3 11.8
Spire Missouri      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return 308.2 380.3 412.5
Spire Missouri | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return 111.4 170.5 159.7
Spire Missouri | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return 185.8 198.5 241.0
Spire Missouri | Other      
Regulatory Assets [Line Items]      
Total Regulatory Assets Not Earning a Return $ 11.0 $ 11.3 $ 11.8