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Stock Incentive Plans and Stock Based Compensation - Additional Information (Detail)
 12 Months Ended 12 Months Ended
Dec. 31, 2011
USD ($)
Year
Dec. 31, 2010
USD ($)
Dec. 31, 2009
USD ($)
Dec. 31, 2011
GBP (£)
Dec. 31, 2010
GBP (£)
Dec. 31, 2011
Stock Incentive Plan 2011
Dec. 31, 2011
Stock Options Granted to Certain Employee
USD ($)
Dec. 31, 2011
Stock Options Granted Prior to June 30, 2009
Dec. 31, 2011
Stock Options Granted after June 30, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Common stock, par       £ 0.50 £ 0.50        
Newly authorized Shares available for award           3,500,000      
Nonqualified stock options granted to an employee 4,445,000           600,000    
Stock option term             10 years    
Stock option exercise price per share $ 2.69           $ 6.35    
Percentage of options that vest on the first anniversary of the grant date             25.00%    
Percentage of options that vest ratably over the subsequent period             75.00%    
Subsequent vesting period after year one             36 months    
Weighted average fair value of the stock options granted $ 8.61 $ 2.21 $ 1.12            
Cash received from the exercise of options $ 5,200,000                
Intrinsic value of options exercised 11,900,000 10,300,000              
Unrecognized stock-based compensation expense related to unvested stock option 36,900,000 9,600,000              
Unrecognized stock-based compensation expense related to unvested stock option, recognition period (in years) 2.5                
Impact on the presentation in the consolidated statement of cash flows relating to excess tax benefits $ 4,199,000 $ 543,000              
Stock options vesting period               3 years 4 years