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Condensed Consolidated Balance Sheets (Parenthetical)
Mar. 31, 2019
£ / shares
shares
Mar. 31, 2019
$ / shares
shares
Dec. 31, 2018
£ / shares
shares
Dec. 31, 2018
$ / shares
shares
Common Stock, Par or Stated Value Per Share | $ / shares   $ 0.10    
Treasury shares, shares held (in shares) 5,900,000 5,900,000 5,900,000 5,900,000
Class A ordinary shares, U.S. [Member]        
Common Stock, Par or Stated Value Per Share | $ / shares   $ 0.4   $ 0.4
Common shares, shares issued (in shares) 115,200,000 115,200,000 115,200,000 115,200,000
Common Class B, Par Value In GBP [Member]        
Common Stock, Par or Stated Value 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