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Debt - 3.692% Senior Unsecured Notes Due 2028 - Additional Information (Detail) - 3.692% Senior Unsecured Notes due 2028 [Member] - GBP (£)
12 Months Ended
Dec. 05, 2019
Dec. 31, 2020
Debt Instrument [Line Items]    
Senior unsecured notes face amount £ 600,000,000  
Percentage of par value on senior notes 99.998%  
Senior unsecured notes, payable term   Interest on the notes is payable on June 5 of each year.
Debt instrument maturity date Jun. 05, 2028