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Stock Compensation Plans - Purchased Restricted Stock Plan - Additional Information (Detail) (PRS [Member], USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
PRS [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Percentage of discount on fair market value on grant date 50.00%    
Number of shares issued 228,750 174,212 213,714
Value of shares purchased by employees $ 6.9 $ 5.4 $ 4.8
Shares purchased by employees 114,375 87,106 106,857
Total fair value of vested 7.1    
Unrecognized compensation cost related to non-vested stock options and SSAR, RSU, PRS and Cash RSU awards granted $ 4.7    
Unrecognized compensation cost related to non-vested stock options and SSAR, RSU, PRS and Cash RSU awards granted, weighted average period, in years 1 year 9 months 18 days