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Debt and Financing Costs - Additional Information (Detail)
1 Months Ended 3 Months Ended 6 Months Ended
Jan. 31, 2017
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
Contract
Jun. 30, 2016
USD ($)
Jun. 30, 2017
GBP (£)
Nov. 30, 2016
USD ($)
Debt Instrument [Line Items]                
Letters of credit outstanding, amount | £             £ 147,500,000  
Loss on extinguishment of debt $ 1,000,000 $ 0   $ 0 $ 1,000,000 $ 0    
Commercial Paper [Member]                
Debt Instrument [Line Items]                
Debt instrument, face amount   3,500,000,000.0     $ 3,500,000,000.0      
Debt instrument term         270 days      
Letter of Credit [Member]                
Debt Instrument [Line Items]                
Number of debt instruments (in contracts) | Contract         3      
Revolving Credit Facility [Member]                
Debt Instrument [Line Items]                
Credit facility maximum borrowing capacity   3,500,000,000.0     $ 3,500,000,000.0      
Credit Facility Matures in June 2020 [Member] | Letter of Credit [Member]                
Debt Instrument [Line Items]                
Line of credit amount   750,000,000     $ 750,000,000      
Credit Facility Matures in June 2020 [Member] | Revolving Credit Facility [Member]                
Debt Instrument [Line Items]                
Line of credit facility, number of extension Options | Contract         2      
Extended financing agreement term         1 year      
Credit Facility Matures in February 2020 [Member] | Letter of Credit [Member]                
Debt Instrument [Line Items]                
Debt instrument, face amount | £             900,000,000  
Credit facility maximum borrowing capacity | £             £ 1,075,000,000.000  
Unsecured Debt [Member]                
Debt Instrument [Line Items]                
Debt instrument, face amount   8,580,000,000 $ 8,650,000,000   $ 8,580,000,000      
Unsecured Debt [Member] | 7% senior notes due February 1, 2018 [Member]                
Debt Instrument [Line Items]                
Debt instrument, face amount   $ 150,000,000     $ 150,000,000      
Debt instrument, stated interest rate   7.00%     7.00%   7.00%  
Senior Notes [Member]                
Debt Instrument [Line Items]                
Repurchased face amount of debt 69,000,000   181,000,000          
Senior notes purchased and canceled 71,000,000   182,000,000          
Debt instrument, premium amount $ 1,000,000   $ 500,000          
Maximum [Member] | Credit Facility Matures in June 2020 [Member] | Revolving Credit Facility [Member]                
Debt Instrument [Line Items]                
Line of credit facility aggregate commitment amount         $ 4,500,000,000.0      
Maximum [Member] | Senior Notes [Member]                
Debt Instrument [Line Items]                
Repurchased face amount of debt               $ 250,000,000