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Partners' Capital (Allocations of Net Income) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Feb. 19, 2020
Nov. 19, 2019
Aug. 14, 2019
May 15, 2019
Feb. 14, 2019
Nov. 14, 2018
Aug. 15, 2018
May 15, 2018
Feb. 14, 2018
Nov. 14, 2017
Aug. 15, 2017
May 16, 2017
Feb. 21, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Schedule of Partners' Capital [Line Items]                                
Distribution Made to Limited Partner, Cash Distributions Paid   $ 68 $ 68 $ 68 $ 68 $ 68 $ 68 $ 68 $ 82 $ 82 $ 82 $ 82 $ 81 $ 353 $ 383 $ 431
Cash distribution per unit (in dollars per share)                           $ 3.3020 $ 3.3020 $ 3.3020
General Partner [Member]                                
Schedule of Partners' Capital [Line Items]                                
Distribution Made to Limited Partner, Cash Distributions Paid   $ 18 $ 18 $ 18 $ 18 $ 18 $ 17 $ 18 $ 21 $ 22 $ 21 $ 21 $ 21      
Common Units [Member]                                
Schedule of Partners' Capital [Line Items]                                
Distributions                           $ 273 $ 272 $ 328
Distributions in excess of net income                           (38) (557) (293)
Limited partners' interest in net income                           $ 235 $ (285) $ 35
Subsequent Event [Member]                                
Schedule of Partners' Capital [Line Items]                                
Distribution Made to Limited Partner, Cash Distributions Paid $ 69                              
Subsequent Event [Member] | General Partner [Member]                                
Schedule of Partners' Capital [Line Items]                                
Distribution Made to Limited Partner, Cash Distributions Paid $ 18