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Condensed Consolidated Balance Sheets (Parenthetical)
Mar. 31, 2018
£ / shares
shares
Mar. 31, 2018
$ / shares
shares
Dec. 31, 2017
£ / shares
shares
Dec. 31, 2017
$ / shares
shares
Treasury shares, shares held (in shares) 9,900,000 9,900,000 11,100,000 11,100,000
Class A ordinary shares, U.S. [Member]        
Common stock, par value per share (in dollars per share or pounds sterling per share) | $ / shares   $ 0.1   $ 0.10
Common shares, shares issued (in shares) 447,100,000 447,100,000 447,100,000 447,100,000
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
Common shares, shares issued (in shares) 50,000 50,000 50,000 50,000