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Consolidated Balance Sheets (Parenthetical)
Dec. 31, 2020
$ / shares
shares
Dec. 31, 2020
£ / shares
shares
Dec. 31, 2019
$ / shares
shares
Dec. 31, 2019
£ / shares
shares
Treasury shares, shares held 6,600,000 6,600,000 7,900,000 7,900,000
Class A Ordinary Shares, U.S. [Member]        
Common shares, par value | $ / shares $ 0.40   $ 0.40  
Common shares, shares issued 206,100,000 206,100,000 205,900,000 205,900,000
Common Class B, Par Value In GBP [Member]        
Common shares, par value | £ / shares   £ 1   £ 1
Common shares, shares issued 50,000 50,000 50,000 50,000