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Exchange Rates - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Sterling [member]      
Disclosure Of Exchange Rates [line items]      
Average rates of exchange for the year 0.75 0.78 0.74
Rates of exchange on the last day of the year 0.78 0.74 0.81
Euro [member]      
Disclosure Of Exchange Rates [line items]      
Average rates of exchange for the year 0.85 0.89 0.90
Rates of exchange on the last day of the year 0.87 0.83 0.95