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Capital Accounts, Issuances of Equity (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Net Cash Proceeds from Sale of Common Units [Abstract]            
Common units issued in connection with the vesting of phantom unit awards, net (in units) 115,665 203,066 3,553,313      
Buyback Program:            
Total of common units repurchased under a buyback program (in units) 3,367,377   709,816      
Common units acquired in connection with buyback program $ 74.9     $ 33.7 $ 88.8 $ 173.8
Long-Term Incentive Plan (2008) [Member]            
Net Cash Proceeds from Sale of Common Units [Abstract]            
Common units issued in connection with the vesting of phantom unit awards, net (in units)         3,872,044  
2019 Buyback Program [Member]            
Buyback Program:            
Amount authorized under 2019 Buyback Program 2,000.0       $ 2,000.0  
Remaining available capacity under the 2019 Buyback Program $ 1,640.0       $ 1,640.0  
Total of common units repurchased under a buyback program (in units) 3,367,377     1,984,507 4,077,193 8,342,246
Common units acquired in connection with buyback program $ 74.9     $ 33.7 $ 88.8 $ 173.8
At-the-Market Registration [Member]            
Registration Statements and Equity Offerings [Line Items]            
Maximum common units authorized for issuance 2,540.0       2,540.0  
Remaining units available for issuance $ 2,540.0       2,540.0  
Employee Unit Purchase Plan [Member]            
Net Cash Proceeds from Sale of Common Units [Abstract]            
Employer contribution to EUPP         $ 2.9  
Distribution Reinvestment and Employee Unit Purchase Plans [Member]            
Net Cash Proceeds from Sale of Common Units [Abstract]            
Number of common units purchased on the open market and delivered to participants (in units)         4,754,016