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Derivative Instruments and Hedging Activities - Open NYMEX Natural Gas Futures Positions (Details) - Spire Missouri - Natural Gas Futures Purchased
MMBTU in Millions
12 Months Ended
Sep. 30, 2020
MMBTU
Derivative [Line Items]  
Notional (MMBtu millions) 25.9
Maximum Term (Months) 12 months