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Share-Based Compensation Plan (Textual) (Details) (USD $)
 12 Months Ended 0 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Stock Options and Restricted Stock Performance Units [Member]
Dec. 31, 2011
Stock Options and Restricted Stock Performance Units [Member]
Dec. 31, 2010
Stock Options and Restricted Stock Performance Units [Member]
Dec. 31, 2012
Stock Options [Member]
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Dec. 31, 2011
Restricted Stock Units (RSUs) [Member]
Dec. 31, 2010
Restricted Stock Units (RSUs) [Member]
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Maximum [Member]
Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Minimum [Member]
Apr. 30, 2010
OAK [Member]
Stock Options [Member]
Dec. 31, 2010
OAK [Member]
Stock Options [Member]
Share-Based Compensation Plans (Textual) [Abstract]                            
Share-based compensation expense $ 1,865,000 $ 1,517,000 $ 1,395,000 $ 1,900,000 $ 1,500,000 $ 1,400,000                
Corporation granted options to purchase common stock             229,763              
Corporation granted options to purchase restricted stock performance units               69,772 52,087 40,629        
Common stock available for future grants under share-based compensation plans 986,095                          
Stock option vesting period             3 years              
Stock option expiration period             10 years              
Weighted-average remaining contractual terms for outstanding stock options             5 years 7 months 6 days              
Weighted-average remaining contractual terms for exercisable stock options             4 years 1 month 6 days              
Intrinsic value of outstanding in-the-money stock options             560,000              
Intrinsic value of exercisable in-the-money stock options             310,000              
Closing price of common stock             $ 23.76              
Unrecognized compensation expense             $ 1,400,000 $ 1,700,000            
Expected weighted average period for unrecognized compensation expense to be recognized             2 years 2 years            
Period of common stock historical volatility             7 years              
Vested stock option conversion ratio                         1.306  
Stock option exercise price conversion ratio                         1.306  
Number of options issued in OAK acquistion                         26,425 26,425
Stock options issued in OAK acquisition, weighted average exercise price per share (USD per share)                           $ 26.83
Restricted stock performance units eligible to vest as a multiple of number of units granted                     1.5 0.5