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Stock Based Compensation Expense - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of Shares, Granted 145,884  
Expenses recognized period   3 years
Fair Value of granted share $ 31.27 $ 20.45
Number of Shares, Forfeited 8,000  
Weighted Average Grant-Date Fair Value, Forfeited $ 22.17  
Granted an option to acquire shares, shares     
Option Vested     
Weighted Average Grant-Date Fair Value, Vested     
Average vest date value Restricted share   $ 17.58
Stock Options
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense $ 209 $ 234
Nonvested unamortized stock-based compensation expense, Value 609  
Nonvested unamortized stock-based compensation expense, contractual term 8 months 12 days  
Restricted Stock
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense 479 188
Nonvested unamortized stock-based compensation expense, Value $ 7,680  
Nonvested unamortized stock-based compensation expense, contractual term 2 years 7 months 6 days  
Number of Shares, Granted 145,884  
Number of shares expected to vested 3,000  
Expenses recognized period 3 years  
Fair Value of granted share $ 31.27  
Number of Shares, Forfeited 8,000  
Weighted Average Grant-Date Fair Value, Forfeited $ 22.17  
Option Vested   82,640
Weighted Average Grant-Date Fair Value, Vested   $ 11.73
Restricted Stock | Cliff Vest Three Years
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Granted an option to acquire shares, shares   207,184