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Stock Incentive Plans and Stock Based Compensation - Additional Information (Detail)
Share data in Millions, except Per Share data, unless otherwise specified
 12 Months Ended 12 Months Ended
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Dec. 31, 2012
GBP (£)
Dec. 31, 2011
GBP (£)
Dec. 31, 2012
Stock Based Compensation Expense
USD ($)
Dec. 31, 2012
Stock Options Granted Prior to June 30, 2009
Dec. 31, 2012
Stock Options Granted after June 30, 2009
Dec. 31, 2012
Stock Incentive Plan 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Common stock, par       £ 0.50 £ 0.50        
Newly authorized Shares available for award                 3.5
Weighted average fair value of the stock options granted $ 8.79 $ 8.61 $ 2.21            
Cash received from the exercise of options $ 8,200,000 $ 5,200,000       $ 8,300,000      
Intrinsic value of options exercised 34,100,000 11,900,000              
Unrecognized stock-based compensation expense related to unvested stock option 41,800,000 36,900,000              
Unrecognized stock-based compensation expense related to unvested stock option, recognition period 2 years 3 months 18 days                
Impact on the presentation in the consolidated statement of cash flows relating to excess tax benefits 11,334,000 4,199,000 543,000            
Compensation expense related to stock option 16,700,000 9,200,000 5,200,000            
Stock options vesting period             3 years 4 years  
Compensation expense in relation to restricted stock units $ 1,400,000 $ 0 $ 0