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Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Total gains (losses) included in:      
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities $ 27.4 $ 79.3 $ (27.2)
Fair Value, Measurements, Recurring [Member]      
Financial assets [Abstract]      
Interest rate derivatives   12.4  
Value before application of CME Rule 814 1,231.3 1,570.1  
Impact of CME Rule 814 (992.8) (1,340.9)  
Total commodity derivatives 238.5 229.2  
Total 238.5 241.6  
Financial liabilities [Abstract]      
Interest rate derivatives   120.1  
Commodity derivatives:      
Value before application of CME Rule 814 1,199.5 2,138.9  
Impact of CME Rule 814 (943.8) (1,892.6)  
Total commodity derivatives 255.7 246.3  
Total 255.7 366.4  
Net value before application of CME Rule 814 to commodity hedging portfolio 31.8    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]      
Financial assets [Abstract]      
Interest rate derivatives   0.0  
Value before application of CME Rule 814 121.9 678.6  
Impact of CME Rule 814 (121.9) (678.6)  
Total commodity derivatives 0.0 0.0  
Total 0.0 0.0  
Financial liabilities [Abstract]      
Interest rate derivatives   0.0  
Commodity derivatives:      
Value before application of CME Rule 814 198.9 1,065.6  
Impact of CME Rule 814 (198.9) (1,065.6)  
Total commodity derivatives 0.0 0.0  
Total 0.0 0.0  
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]      
Financial assets [Abstract]      
Interest rate derivatives   12.4  
Value before application of CME Rule 814 1,109.4 878.6  
Impact of CME Rule 814 (870.9) (650.4)  
Total commodity derivatives 238.5 228.2  
Total 238.5 240.6  
Financial liabilities [Abstract]      
Interest rate derivatives   120.1  
Commodity derivatives:      
Value before application of CME Rule 814 1,000.3 1,047.4  
Impact of CME Rule 814 (744.9) (807.3)  
Total commodity derivatives 255.4 240.1  
Total 255.4 360.2  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]      
Financial assets [Abstract]      
Interest rate derivatives   0.0  
Value before application of CME Rule 814 0.0 12.9  
Impact of CME Rule 814 0.0 (11.9)  
Total commodity derivatives 0.0 1.0  
Total 0.0 1.0  
Financial liabilities [Abstract]      
Interest rate derivatives   0.0  
Commodity derivatives:      
Value before application of CME Rule 814 0.3 25.9  
Impact of CME Rule 814 0.0 (19.7)  
Total commodity derivatives 0.3 6.2  
Total 0.3 6.2  
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]      
Financial liability balance, net, beginning of period (5.2) (509.8)  
Total gains (losses) included in:      
Transfer out of Level 3 - Liquidity Option [1] 0.0 511.9  
Other transfers out of Level 3 5.8 17.1  
Financial liability balance, net, end of period (0.3) (5.2) $ (509.8)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities (0.8) 0.7  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]      
Total gains (losses) included in:      
Other comprehensive income (loss) (0.2) (23.7)  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]      
Total gains (losses) included in:      
Net income [2] 0.7 (1.6)  
Settlements [2] (1.4) 3.2  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]      
Total gains (losses) included in:      
Net income $ 0.0 $ (2.3)  
[1] In March 2020, the Liquidity Option settled and was transferred out of Level 3.  See Note 8 for information regarding the Liquidity Option.
[2] There were unrealized losses of $0.8 million and unrealized gains of $0.7 million included in these amounts for the years ended December 31, 2021 and 2020, respectively.