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Share-Based Compensation Plans - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Age
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Restricted stock grants [Member]
Dec. 31, 2011
Restricted stock grants [Member]
Dec. 31, 2010
Restricted stock grants [Member]
Dec. 31, 2012
Restricted stock grants [Member]
Executive Officers [Member]
Dec. 31, 2012
Restricted stock grants [Member]
Non-employee members of board of directors [Member]
Dec. 31, 2011
Restricted stock grants [Member]
Non-employee members of board of directors [Member]
Dec. 31, 2010
Restricted stock grants [Member]
Non-employee members of board of directors [Member]
Dec. 31, 2012
Incentive Plan [Member]
Sep. 27, 2008
Incentive Plan [Member]
Dec. 31, 2012
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Common stock authorized                     9,847,000 4,200,000 1,050,000
Percentage increase in the number of shares authorized for issuance under the Incentive Plan                     2.50%    
Shares granted       563,000 6,000 4,000 552,000 11,000 6,000 4,000      
Percentage of shares to vest each year             20.00%            
Vesting period 5 years           5 years            
Unrecognized share-based compensation cost $ 11,280     $ 7,531                  
Costs are expected to be recognized over a weighted average period 1 year 7 months 6 days     2 years 7 months 6 days                  
Expiration period of option vested 10 years                        
Cash received from stock option exercises 8,049 1,681 1,747                    
Dividend yield assumption 0.00% 0.00% 0.00%                    
University made discretionary matching contributions $ 1,500 $ 933 $ 745                    
Minimum eligible age to participate in the plan 21