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Consolidated Balance Sheets (Parenthetical)
Dec. 31, 2015
£ / shares
shares
Dec. 31, 2015
$ / shares
shares
Dec. 31, 2014
£ / shares
shares
Dec. 31, 2014
$ / shares
shares
Treasury shares, shares held 7,800,000 7,800,000 6,500,000 6,500,000
Class A Ordinary Shares, U.S. [Member]        
Common shares, par value | $ / shares   $ 0.10   $ 0.10
Common shares, shares authorized 450,000,000 450,000,000 450,000,000.0 450,000,000.0
Common shares, shares issued 243,100,000 243,100,000 240,700,000 240,700,000
Common Class B, Par Value In GBP [Member]        
Common shares, par value | £ / shares £ 1   £ 1  
Common shares, shares authorized 50,000 50,000 50,000 50,000
Common shares, shares issued 50,000 50,000 50,000 50,000