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Summary of Significant Accounting Policies - Redeemable Noncontrolling Interest (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Noncontrolling Interest [Line Items]        
Balance as of beginning of period     $ 78  
Distributions to noncontrolling interest     (34)  
Contributions from redeemable noncontrolling interest     26  
Comprehensive loss attributable to noncontrolling interest $ 26 $ (17) (12) $ (56)
Balance as of end of period $ 69   69  
Redeemable noncontrolling interest        
Noncontrolling Interest [Line Items]        
Distributions to noncontrolling interest     (2)  
Non-cash adjustments to noncontrolling interest     (9)  
Comprehensive loss attributable to noncontrolling interest     $ (24)