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Currency Conversion - Additional Information (Detail)
6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
US dollar to Pound sterling [member]      
Disclosure Of Foreign Currency Translation [line items]      
Average exchange rate 1.2938 1.3763 1.3351
Closing exchange rate 1.2695 1.3194 1.2746
Euro to Pound sterling [member]      
Disclosure Of Foreign Currency Translation [line items]      
Average exchange rate 1.1453 1.1366 1.1303
Closing exchange rate 1.1175 1.1299 1.1130