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Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Total gains (losses) included in:      
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities $ 96.3 $ (136.9)  
Fair Value, Measurements, Recurring [Member]      
Financial assets [Abstract]      
Value before application of CME Rule 814 309.3   $ 456.9
Impact of CME Rule 814 (170.9)   (295.0)
Total commodity derivatives 138.4   161.9
Total 138.4   161.9
Financial liabilities [Abstract]      
Liquidity option agreement 447.8   390.0
Commodity derivatives:      
Value before application of CME Rule 814 244.0   398.1
Impact of CME Rule 814 (135.6)   (235.7)
Total commodity derivatives 108.4   162.4
Total 556.2   552.4
Net value before application of CME Rule 814 to commodity hedging portfolio 65.3    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]      
Financial assets [Abstract]      
Value before application of CME Rule 814 82.9   172.3
Impact of CME Rule 814 (47.9)   (134.8)
Total commodity derivatives 35.0   37.5
Total 35.0   37.5
Financial liabilities [Abstract]      
Liquidity option agreement 0.0   0.0
Commodity derivatives:      
Value before application of CME Rule 814 87.6   85.5
Impact of CME Rule 814 (53.1)   (48.6)
Total commodity derivatives 34.5   36.9
Total 34.5   36.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]      
Financial assets [Abstract]      
Value before application of CME Rule 814 219.9   282.4
Impact of CME Rule 814 (120.4)   (159.3)
Total commodity derivatives 99.5   123.1
Total 99.5   123.1
Financial liabilities [Abstract]      
Liquidity option agreement 0.0   0.0
Commodity derivatives:      
Value before application of CME Rule 814 145.5   291.2
Impact of CME Rule 814 (74.6)   (172.9)
Total commodity derivatives 70.9   118.3
Total 70.9   118.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]      
Financial assets [Abstract]      
Value before application of CME Rule 814 6.5   2.2
Impact of CME Rule 814 (2.6)   (0.9)
Total commodity derivatives 3.9   1.3
Total 3.9   1.3
Financial liabilities [Abstract]      
Liquidity option agreement 447.8   390.0
Commodity derivatives:      
Value before application of CME Rule 814 10.9   21.4
Impact of CME Rule 814 (7.9)   (14.2)
Total commodity derivatives 3.0   7.2
Total 450.8   $ 397.2
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]      
Financial liability balance, net, beginning of period (395.9) (332.7)  
Total gains (losses) included in:      
Transfers out of Level 3 (0.2) 0.0  
Financial liability balance, net, end of period (446.9) (341.9)  
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities 3.0 (1.7)  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]      
Total gains (losses) included in:      
Net income [1] 3.1 (0.5)  
Settlements [1] (0.1) (1.2)  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]      
Total gains (losses) included in:      
Net income (57.8) (7.5)  
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]      
Total gains (losses) included in:      
Other comprehensive income $ 4.0 $ 0.0  
[1] There were unrealized gains of $3.0 million and unrealized losses of $1.7 million included in these amounts for the three months ended March 31, 2019 and 2018, respectively.