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Condensed Consolidated Balance Sheets (Parenthetical)
Mar. 31, 2021
$ / shares
shares
Mar. 31, 2021
£ / shares
shares
Dec. 31, 2020
$ / shares
shares
Dec. 31, 2020
£ / shares
shares
Mar. 31, 2020
shares
Treasury shares, shares held (in shares) 6,400,000 6,400,000 6,600,000 6,600,000  
Class A ordinary shares, U.S.          
Common stock, par value per share (in dollars per share or pounds sterling per share) | $ / shares $ 0.40   $ 0.40    
Common shares, shares issued (in shares)     206,100,000 206,100,000 206,100,000
Common Class B, Par Value In GBP [Member]          
Common stock, par value per share (in dollars per share or pounds sterling per share) | £ / shares   £ 1   £ 1  
Common shares, shares authorized (in shares) 50,000 50,000 50,000 50,000