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Debt - Senior Unsecured Notes (Details) - USD ($)
Jul. 27, 2017
May 01, 2017
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument        
Principal balance on debt     $ 7,917,236,000  
Senior unsecured notes     6,398,976,000 $ 6,396,451,000
Unsecured Debt        
Debt Instrument        
Principal balance on debt     6,450,000,000  
Senior unsecured notes     $ 0 $ 0
Unsecured Note 5.625%        
Debt Instrument        
Repayment of senior unsecured notes   $ 250,000,000    
Percentage of stated interest rate   5.625%    
Unsecured Note 5.375%        
Debt Instrument        
Repayment of senior unsecured notes $ 500,000,000      
Percentage of stated interest rate 5.375%