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Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 01, 2018
Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Increase in investments in unconsolidated entities $ 2,756 $ 2,343 $ 2,027    
Increase in partners' capital 12,358 10,707 9,234   $ 9,381
Cash outflow reclassed from operating activities 2,496 718 1,347    
Cash outflow reclassed to financing activities $ 940 (571) (813)    
ASU 2016-09          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cash outflow reclassed from operating activities   7 14    
Cash outflow reclassed to financing activities   $ 7 $ 14    
Subsequent Event | ASU 2017-05          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Increase in investments in unconsolidated entities       $ 110  
Increase in partners' capital       $ 110