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Debt - Bank Line of Credit and Term Loans (Details)
£ in Millions
1 Months Ended 12 Months Ended
Oct. 19, 2017
USD ($)
renewal_option
Jun. 30, 2017
GBP (£)
Mar. 31, 2017
GBP (£)
Jan. 31, 2015
Dec. 31, 2017
USD ($)
Dec. 31, 2017
GBP (£)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]              
Bank line of credit         $ 1,017,076,000   $ 899,718,000
Term loans         $ 228,288,000   $ 440,062,000
Bank Line of Credit              
Debt Instrument [Line Items]              
Line of credit facility, maximum borrowing capacity $ 2,000,000,000.0            
Debt Instrument, Number of Extensions | renewal_option 2            
Loan, basis spread on variable rate         1.00%    
Debt instrument, facility fee (as a percent)         0.20%    
Line of credit facility additional aggregate amount, maximum         $ 750,000,000    
Bank line of credit         1,000,000,000    
Line of credit, portion denominated in GBP | £           £ 105  
Term loans, portion denominated in GBP         $ 142,000,000    
Weighted-average interest rate (as a percent)         2.74% 2.74%  
Length of debt instrument extension period 6 months            
2012 Term Loan              
Debt Instrument [Line Items]              
Repayments of debt | £     £ 137        
2015 Term Loan              
Debt Instrument [Line Items]              
Term loans, portion denominated in GBP | £           £ 169  
Length of debt instrument extension period       1 year      
Repayments of debt | £   £ 51          
Term of facility       4 years      
Term of contract       3 years      
Term loans         $ 229,000,000    
2015 Term Loan | GBP LIBOR              
Debt Instrument [Line Items]              
Loan, basis spread on variable rate   1.15%          
Line of Credit and Term Loan              
Debt Instrument [Line Items]              
Debt instrument, covenant debt to assets (as a percent)         60.00% 60.00%  
Debt instrument, covenant secured debt to assets (as a percent)         30.00% 30.00%  
Debt instrument, covenant unsecured debt to unencumbered assets (as a percent)         60.00% 60.00%  
Debt instrument, covenant minimum fixed charge coverage ratio         1.5 1.5  
Debt instrument, covenant net worth         $ 6,500,000,000