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Financial Instruments - Summary of Margin over LIBOR Depending on Leverage Ration (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 3.85 4.22 5.21
Greater than 5.50x [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 3.50%    
Less than 5.00 Greater than 4.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 3.00%    
Less than 4.50 Greater than 4.00 [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 2.50%    
Less than 4.00 Greater than 3.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 2.125%    
Less than 3.50 Greater than 3.00 [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 1.75%    
Less than 3.00 Greater than 2.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 1.50%    
Less than 2.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Applicable margin 1.25%    
Top of range [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 3.75    
Top of range [member] | Greater than 5.50x [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 5.50    
Top of range [member] | Less than 5.00 Greater than 4.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 5.00    
Top of range [member] | Less than 4.50 Greater than 4.00 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 4.50    
Top of range [member] | Less than 4.00 Greater than 3.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 4.00    
Top of range [member] | Less than 3.50 Greater than 3.00 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 3.50    
Top of range [member] | Less than 3.00 Greater than 2.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 3.00    
Bottom of range [member] | Less than 5.00 Greater than 4.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 4.50    
Bottom of range [member] | Less than 4.50 Greater than 4.00 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 4.00    
Bottom of range [member] | Less than 4.00 Greater than 3.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 3.50    
Bottom of range [member] | Less than 3.50 Greater than 3.00 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 3.00    
Bottom of range [member] | Less than 3.00 Greater than 2.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 2.50    
Bottom of range [member] | Less than 2.50 [member]      
Disclosure of detailed information about financial instruments [line items]      
Consolidated leverage ratio 2.50