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Long-Term Debt - Additional Information (Details)
1 Months Ended
Oct. 31, 2016
USD ($)
May 31, 2016
USD ($)
extension
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2014
USD ($)
Nov. 30, 2012
USD ($)
Debt Instrument [Line Items]            
Borrowings outstanding     $ 8,832,000,000 $ 8,838,000,000    
Senior Notes            
Debt Instrument [Line Items]            
Face amount of notes         $ 6,000,000,000.0 $ 3,000,000,000.0
Borrowings outstanding     8,300,000,000      
Unamortized discount     86,000,000 90,000,000    
Estimated fair value of notes     8,900,000,000 8,700,000,000    
Line of Credit and Other Long-term Debt            
Debt Instrument [Line Items]            
Borrowings outstanding     582,000,000 588,000,000    
Credit Facility | Revolving Credit Facility            
Debt Instrument [Line Items]            
Borrowings outstanding     500,000,000 495,000,000    
Credit Facility | Revolving Credit Facility | October 2016 Revolving Credit Facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity $ 500,000,000          
Credit term 3 years          
Commitment fee percentage 0.50%          
Borrowings outstanding     $ 500,000,000 $ 495,000,000    
Weighted average interest rate     2.50% 2.30%    
Collateral amount     $ 589,000,000 $ 579,000,000    
Credit Facility | Revolving Credit Facility | October 2016 Revolving Credit Facility | LIBOR            
Debt Instrument [Line Items]            
Basis spread on variable rate (as a percent) 1.65%          
Credit Facility | Revolving Credit Facility | May 2016 Revolving Credit Facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 3,000,000,000        
Credit term   3 years        
Commitment fee percentage   0.05%        
Borrowings outstanding     0 0    
Credit Agreement, number of extensions | extension   3        
Credit Agreement, additional term   1 year        
Credit Facility | Revolving Credit Facility | May 2016 Revolving Credit Facility | Downgraded Credit Ratings            
Debt Instrument [Line Items]            
Commitment fee percentage   0.09%        
Credit Facility | Revolving Credit Facility | May 2016 Revolving Credit Facility | LIBOR            
Debt Instrument [Line Items]            
Basis spread on variable rate (as a percent)   0.60%        
Credit Facility | Revolving Credit Facility | May 2016 Revolving Credit Facility | LIBOR | Downgraded Credit Ratings            
Debt Instrument [Line Items]            
Basis spread on variable rate (as a percent)   1.00%        
Other Long-term Debt            
Debt Instrument [Line Items]            
Borrowings outstanding     $ 82,000,000 $ 93,000,000    
Weighted average interest rate     3.90% 3.40%