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Condensed Consolidated Balance Sheets (Parenthetical)
Jun. 30, 2018
£ / shares
shares
Jun. 30, 2018
$ / shares
shares
Dec. 31, 2017
£ / shares
shares
Dec. 31, 2017
$ / shares
shares
Treasury shares, shares held (in shares) 23,600,000 23,600,000 11,100,000 11,100,000
Class A ordinary shares, U.S.        
Common stock, par value per share (in dollars per share or pounds sterling per share) | $ / shares   $ 0.10   $ 0.10
Common shares, shares issued (in shares) 460,700,000 460,700,000 447,100,000 447,100,000
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 issued (in shares) 50,000 50,000 50,000 50,000
Common shares, shares authorized (in shares) 50,000 50,000 50,000 50,000