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Accounting policies - Summary of Estimated Useful Life of Intangible Assets (Detail)
 12 Months Ended
Dec. 31, 2018
Bottom of range [member] | Brand names [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 10 years
Bottom of range [member] | Customer-related intangibles [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 3 years
Bottom of range [member] | Other proprietary tools [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 3 years
Bottom of range [member] | Other [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 3 years
Top of range [member] | Brand names [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 20 years
Top of range [member] | Customer-related intangibles [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 10 years
Top of range [member] | Other proprietary tools [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 10 years
Top of range [member] | Other [member]  
Disclosure of detailed information about intangible assets [line items]  
Intangible assets, estimated useful life 5 years