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Investment Properties - Investment Properties Depreciated on a Straight-Line Basis Over Useful Lives (Detail)
12 Months Ended
Dec. 31, 2018
Bottom of range [member] | Main plant buildings [Member]  
Disclosure of detailed information about investment property [line items]  
Investment properties are depreciated using the straight-line method over their estimated useful lives 10 years
Bottom of range [member] | Other Investment Properties [Member]  
Disclosure of detailed information about investment property [line items]  
Investment properties are depreciated using the straight-line method over their estimated useful lives 3 years
Top of range [member] | Main plant buildings [Member]  
Disclosure of detailed information about investment property [line items]  
Investment properties are depreciated using the straight-line method over their estimated useful lives 40 years
Top of range [member] | Other Investment Properties [Member]  
Disclosure of detailed information about investment property [line items]  
Investment properties are depreciated using the straight-line method over their estimated useful lives 20 years