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Debt and Financing Costs - Additional Information (Detail)
1 Months Ended 3 Months Ended
Jan. 31, 2017
USD ($)
Jun. 30, 2015
Mar. 31, 2017
USD ($)
Contract
Dec. 31, 2016
USD ($)
Mar. 31, 2016
USD ($)
Mar. 31, 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    
Commercial Paper [Member]              
Debt Instrument [Line Items]              
Debt instrument, face amount     $ 3,500,000,000.0        
Debt instrument term     270 days        
Letter of Credit [Member]              
Debt Instrument [Line Items]              
Debt Instrument, Number | Contract     3        
Revolving Credit Facility [Member]              
Debt Instrument [Line Items]              
Credit facility maximum borrowing capacity     $ 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        
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  
Extended financing agreement term     1 year        
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      
Unsecured Debt [Member] | 7% senior notes due February 1, 2018 [Member]              
Debt Instrument [Line Items]              
Debt instrument, face amount     $ 150,000,000        
Debt instrument, stated interest rate     7.00%     7.00%  
Senior Notes [Member]              
Debt Instrument [Line Items]              
Debt Instrument, Repurchased Face Amount $ 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]              
Debt Instrument, Repurchased Face Amount             $ 250,000,000