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Risk Management Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Risk Management Assets Current    
Derivatives, Fair Value [Line Items]    
Risk management assets $ 27.9 $ 10.4
Risk Management Assets Noncurrent    
Derivatives, Fair Value [Line Items]    
Risk management assets 18.8 2.8
Risk Management Liabilities Current    
Derivatives, Fair Value [Line Items]    
Risk management liabilities 47.0 78.2
Risk Management Liabilities Noncurrent    
Derivatives, Fair Value [Line Items]    
Risk management liabilities 75.7 144.6
Interest Rate Risk | Risk Management Assets Current    
Derivatives, Fair Value [Line Items]    
Risk management assets 0.0 0.0
Interest Rate Risk | Risk Management Assets Noncurrent    
Derivatives, Fair Value [Line Items]    
Risk management assets 0.0 0.0
Interest Rate Risk | Risk Management Liabilities Current    
Derivatives, Fair Value [Line Items]    
Risk management liabilities 46.6 70.9
Interest Rate Risk | Risk Management Liabilities Noncurrent    
Derivatives, Fair Value [Line Items]    
Risk management liabilities 68.8 99.5
Commodity Price Risk Programs | Risk Management Assets Current    
Derivatives, Fair Value [Line Items]    
Risk management assets 27.9 10.4
Commodity Price Risk Programs | Risk Management Assets Noncurrent    
Derivatives, Fair Value [Line Items]    
Risk management assets 18.8 2.8
Commodity Price Risk Programs | Risk Management Liabilities Current    
Derivatives, Fair Value [Line Items]    
Risk management liabilities 0.4 7.3
Commodity Price Risk Programs | Risk Management Liabilities Noncurrent    
Derivatives, Fair Value [Line Items]    
Risk management liabilities $ 6.9 $ 45.1