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Currency conversion - Additional Information (Detail)
6 Months Ended 12 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
US dollar to Pound sterling [member]      
Disclosure Of Foreign Currency Translation [line items]      
Average exchange rate 1.2604 1.2938 1.2765
Closing exchange rate 1.2379 1.2695 1.3263
Euro to Pound sterling [member]      
Disclosure Of Foreign Currency Translation [line items]      
Average exchange rate 1.1439 1.1453 1.1403
Closing exchange rate 1.1010 1.1175 1.1813