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Long-Term Debt and Short-Term Borrowings - Revolving Credit Facility - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]      
Letters of credit outstanding amount $ 69,400,000    
2017 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement $ 2,800,000,000    
2017 Credit Agreement [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Credit Agreement applicable fixed rate spread 0.875%    
Credit Agreement applicable daily rate spread 0.00%    
2017 Credit Agreement [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Credit Agreement applicable fixed rate spread 1.25%    
Credit Agreement applicable daily rate spread 0.25%    
2017 Credit Agreement [Member] | Canadian, Australian and New Zealand subsidiaries [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement $ 200,000,000    
2017 Credit Agreement [Member] | U.K. subsidiaries [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement 300,000,000    
2015 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement $ 2,800,000,000    
Revolving credit sub-facilities current borrowings capacity foreign currency fluctuation provision 5.00%    
2015 Credit Agreement [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Credit Agreement applicable fixed rate spread 0.85%    
2015 Credit Agreement [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Credit Agreement applicable fixed rate spread 1.00%    
2015 Credit Agreement [Member] | U.K. subsidiaries [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement $ 300,000,000    
2015 Credit Agreement [Member] | Canadian subsidiaries [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement 75,000,000    
2015 Credit Agreement [Member] | Australian and New Zealand subsidiaries [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement 100,000,000    
Revolving credit facility [Member] | 2017 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Letters of credit outstanding amount 0    
Revolving credit facility principal amount outstanding $ 2,000,000    
Revolving credit facility [Member] | 2017 Credit Agreement [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Credit Agreement applicable fixed rate spread 0.775%    
Credit Agreement applicable daily rate spread 0.00%    
Revolving credit facility [Member] | 2017 Credit Agreement [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Credit Agreement applicable fixed rate spread 1.075%    
Credit Agreement applicable daily rate spread 0.075%    
Revolving credit facility [Member] | 2015 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Amounts available to borrow under Credit Agreement $ 2,800,000,000   $ 2,600,000,000
Letters of credit outstanding amount   $ 2,000,000  
Revolving credit facility principal amount outstanding   $ 0